legalization of weed research paper

• Research article
• Open access
• Published: 04 February 2020

Marijuana legalization and historical trends in marijuana use among US residents aged 12–25: results from the 1979–2016 National Survey on drug use and health

• Xinguang Chen 1 ,
• Xiangfan Chen 2 &
• Hong Yan 2  

BMC Public Health volume  20 , Article number:  156 ( 2020 ) Cite this article

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Marijuana is the most commonly used illicit drug in the United States. More and more states legalized medical and recreational marijuana use. Adolescents and emerging adults are at high risk for marijuana use. This ecological study aims to examine historical trends in marijuana use among youth along with marijuana legalization.

Data ( n  = 749,152) were from the 31-wave National Survey on Drug Use and Health (NSDUH), 1979–2016. Current marijuana use, if use marijuana in the past 30 days, was used as outcome variable. Age was measured as the chronological age self-reported by the participants, period was the year when the survey was conducted, and cohort was estimated as period subtracted age. Rate of current marijuana use was decomposed into independent age, period and cohort effects using the hierarchical age-period-cohort (HAPC) model.

After controlling for age, cohort and other covariates, the estimated period effect indicated declines in marijuana use in 1979–1992 and 2001–2006, and increases in 1992–2001 and 2006–2016. The period effect was positively and significantly associated with the proportion of people covered by Medical Marijuana Laws (MML) (correlation coefficients: 0.89 for total sample, 0.81 for males and 0.93 for females, all three p values < 0.01), but was not significantly associated with the Recreational Marijuana Laws (RML). The estimated cohort effect showed a historical decline in marijuana use in those who were born in 1954–1972, a sudden increase in 1972–1984, followed by a decline in 1984–2003.

The model derived trends in marijuana use were coincident with the laws and regulations on marijuana and other drugs in the United States since the 1950s. With more states legalizing marijuana use in the United States, emphasizing responsible use would be essential to protect youth from using marijuana.

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Introduction

Marijuana use and laws in the united states.

Marijuana is one of the most commonly used drugs in the United States (US) [ 1 ]. In 2015, 8.3% of the US population aged 12 years and older used marijuana in the past month; 16.4% of adolescents aged 12–17 years used in lifetime and 7.0% used in the past month [ 2 ]. The effects of marijuana on a person’s health are mixed. Despite potential benefits (e.g., relieve pain) [ 3 ], using marijuana is associated with a number of adverse effects, particularly among adolescents. Typical adverse effects include impaired short-term memory, cognitive impairment, diminished life satisfaction, and increased risk of using other substances [ 4 ].

Since 1937 when the Marijuana Tax Act was issued, a series of federal laws have been subsequently enacted to regulate marijuana use, including the Boggs Act (1952), Narcotics Control Act (1956), Controlled Substance Act (1970), and Anti-Drug Abuse Act (1986) [ 5 , 6 ]. These laws regulated the sale, possession, use, and cultivation of marijuana [ 6 ]. For example, the Boggs Act increased the punishment of marijuana possession, and the Controlled Substance Act categorized the marijuana into the Schedule I Drugs which have a high potential for abuse, no medical use, and not safe to use without medical supervision [ 5 , 6 ]. These federal laws may have contributed to changes in the historical trend of marijuana use among youth.

Movements to decriminalize and legalize marijuana use

Starting in the late 1960s, marijuana decriminalization became a movement, advocating reformation of federal laws regulating marijuana [ 7 ]. As a result, 11 US states had taken measures to decriminalize marijuana use by reducing the penalty of possession of small amount of marijuana [ 7 ].

The legalization of marijuana started in 1993 when Surgeon General Elder proposed to study marijuana legalization [ 8 ]. California was the first state that passed Medical Marijuana Laws (MML) in 1996 [ 9 ]. After California, more and more states established laws permitting marijuana use for medical and/or recreational purposes. To date, 33 states and the District of Columbia have established MML, including 11 states with recreational marijuana laws (RML) [ 9 ]. Compared with the legalization of marijuana use in the European countries which were more divided that many of them have medical marijuana registered as a treatment option with few having legalized recreational use [ 10 , 11 , 12 , 13 ], the legalization of marijuana in the US were more mixed with 11 states legalized medical and recreational use consecutively, such as California, Nevada, Washington, etc. These state laws may alter people’s attitudes and behaviors, finally may lead to the increased risk of marijuana use, particularly among young people [ 13 ]. Reported studies indicate that state marijuana laws were associated with increases in acceptance of and accessibility to marijuana, declines in perceived harm, and formation of new norms supporting marijuana use [ 14 ].

Marijuana harm to adolescents and young adults

Adolescents and young adults constitute a large proportion of the US population. Data from the US Census Bureau indicate that approximately 60 million of the US population are in the 12–25 years age range [ 15 ]. These people are vulnerable to drugs, including marijuana [ 16 ]. Marijuana is more prevalent among people in this age range than in other ages [ 17 ]. One well-known factor for explaining the marijuana use among people in this age range is the theory of imbalanced cognitive and physical development [ 4 ]. The delayed brain development of youth reduces their capability to cognitively process social, emotional and incentive events against risk behaviors, such as marijuana use [ 18 ]. Understanding the impact of marijuana laws on marijuana use among this population with a historical perspective is of great legal, social and public health significance.

Inconsistent results regarding the impact of marijuana laws on marijuana use

A number of studies have examined the impact of marijuana laws on marijuana use across the world, but reported inconsistent results [ 13 ]. Some studies reported no association between marijuana laws and marijuana use [ 14 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ], some reported a protective effect of the laws against marijuana use [ 24 , 26 ], some reported mixed effects [ 27 , 28 ], while some others reported a risk effect that marijuana laws increased marijuana use [ 29 , 30 ]. Despite much information, our review of these reported studies revealed several limitations. First of all, these studies often targeted a short time span, ignoring the long period trend before marijuana legalization. Despite the fact that marijuana laws enact in a specific year, the process of legalization often lasts for several years. Individuals may have already changed their attitudes and behaviors before the year when the law is enacted. Therefore, it may not be valid when comparing marijuana use before and after the year at a single time point when the law is enacted and ignoring the secular historical trend [ 19 , 30 , 31 ]. Second, many studies adapted the difference-in-difference analytical approach designated for analyzing randomized controlled trials. No US state is randomized to legalize the marijuana laws, and no state can be established as controls. Thus, the impact of laws cannot be efficiently detected using this approach. Third, since marijuana legalization is a public process, and the information of marijuana legalization in one state can be easily spread to states without the marijuana laws. The information diffusion cannot be ruled out, reducing the validity of the non-marijuana law states as the controls to compare the between-state differences [ 31 ].

Alternatively, evidence derived based on a historical perspective may provide new information regarding the impact of laws and regulations on marijuana use, including state marijuana laws in the past two decades. Marijuana users may stop using to comply with the laws/regulations, while non-marijuana users may start to use if marijuana is legal. Data from several studies with national data since 1996 demonstrate that attitudes, beliefs, perceptions, and use of marijuana among people in the US were associated with state marijuana laws [ 29 , 32 ].

Age-period-cohort modeling: looking into the past with recent data

To investigate historical trends over a long period, including the time period with no data, we can use the classic age-period-cohort modeling (APC) approach. The APC model can successfully discompose the rate or prevalence of marijuana use into independent age, period and cohort effects [ 33 , 34 ]. Age effect refers to the risk associated with the aging process, including the biological and social accumulation process. Period effect is risk associated with the external environmental events in specific years that exert effect on all age groups, representing the unbiased historical trend of marijuana use which controlling for the influences from age and birth cohort. Cohort effect refers to the risk associated with the specific year of birth. A typical example is that people born in 2011 in Fukushima, Japan may have greater risk of cancer due to the nuclear disaster [ 35 ], so a person aged 80 in 2091 contains the information of cancer risk in 2011 when he/she was born. Similarly, a participant aged 25 in 1979 contains information on the risk of marijuana use 25 years ago in 1954 when that person was born. With this method, we can describe historical trends of marijuana use using information stored by participants in older ages [ 33 ]. The estimated period and cohort effects can be used to present the unbiased historical trend of specific topics, including marijuana use [ 34 , 36 , 37 , 38 ]. Furthermore, the newly established hierarchical APC (HAPC) modeling is capable of analyzing individual-level data to provide more precise measures of historical trends [ 33 ]. The HAPC model has been used in various fields, including social and behavioral science, and public health [ 39 , 40 ].

Several studies have investigated marijuana use with APC modeling method [ 17 , 41 , 42 ]. However, these studies covered only a small portion of the decades with state marijuana legalization [ 17 , 42 ]. For example, the study conducted by Miech and colleagues only covered periods from 1985 to 2009 [ 17 ]. Among these studies, one focused on a longer state marijuana legalization period, but did not provide detailed information regarding the impact of marijuana laws because the survey was every 5 years and researchers used a large 5-year age group which leads to a wide 10-year birth cohort. The averaging of the cohort effects in 10 years could reduce the capability of detecting sensitive changes of marijuana use corresponding to the historical events [ 41 ].

Purpose of the study

In this study, we examined the historical trends in marijuana use among youth using HAPC modeling to obtain the period and cohort effects. These two effects provide unbiased and independent information to characterize historical trends in marijuana use after controlling for age and other covariates. We conceptually linked the model-derived time trends to both federal and state laws/regulations regarding marijuana and other drug use in 1954–2016. The ultimate goal is to provide evidence informing federal and state legislation and public health decision-making to promote responsible marijuana use and to protect young people from marijuana use-related adverse consequences.

Materials and methods

Data sources and study population.

Data were derived from 31 waves of National Survey on Drug Use and Health (NSDUH), 1979–2016. NSDUH is a multi-year cross-sectional survey program sponsored by the Substance Abuse and Mental Health Services Administration. The survey was conducted every 3 years before 1990, and annually thereafter. The aim is to provide data on the use of tobacco, alcohol, illicit drug and mental health among the US population.

Survey participants were noninstitutionalized US civilians 12 years of age and older. Participants were recruited by NSDUH using a multi-stage clustered random sampling method. Several changes were made to the NSDUH after its establishment [ 43 ]. First, the name of the survey was changed from the National Household Survey on Drug Abuse (NHSDA) to NSDUH in 2002. Second, starting in 2002, adolescent participants receive $30 as incentives to improve the response rate. Third, survey mode was changed from personal interviews with self-enumerated answer sheets (before 1999) to the computer-assisted person interviews (CAPI) and audio computer-assisted self-interviews (ACASI) (since 1999). These changes may confound the historical trends [ 43 ], thus we used two dummy variables as covariates, one for the survey mode change in 1999 and another for the survey method change in 2002 to control for potential confounding effect.

Data acquisition

Data were downloaded from the designated website ( https://nsduhweb.rti.org/respweb/homepage.cfm ). A database was used to store and merge the data by year for analysis. Among all participants, data for those aged 12–25 years ( n  = 749,152) were included. We excluded participants aged 26 and older because the public data did not provide information on single or two-year age that was needed for HAPC modeling (details see statistical analysis section). We obtained approval from the Institutional Review Board at the University of Florida to conduct this study.

Variables and measurements

Current marijuana use: the dependent variable. Participants were defined as current marijuana users if they reported marijuana use within the past 30 days. We used the variable harmonization method to create a comparable measure across 31-wave NSDUH data [ 44 ]. Slightly different questions were used in NSDUH. In 1979–1993, participants were asked: “When was the most recent time that you used marijuana or hash?” Starting in 1994, the question was changed to “How long has it been since you last used marijuana or hashish?” To harmonize the marijuana use variable, participants were coded as current marijuana users if their response to the question indicated the last time to use marijuana was within past 30 days.

Chronological age, time period and birth cohort were the predictors. (1) Chronological age in years was measured with participants’ age at the survey. APC modeling requires the same age measure for all participants [ 33 ]. Since no data by single-year age were available for participants older than 21, we grouped all participants into two-year age groups. A total of 7 age groups, 12–13, ..., 24–25 were used. (2) Time period was measured with the year when the survey was conducted, including 1979, 1982, 1985, 1988, 1990, 1991... 2016. (3). Birth cohort was the year of birth, and it was measured by subtracting age from the survey year.

The proportion of people covered by MML: This variable was created by dividing the population in all states with MML over the total US population. The proportion was computed by year from 1996 when California first passed the MML to 2016 when a total of 29 states legalized medical marijuana use. The estimated proportion ranged from 12% in 1996 to 61% in 2016. The proportion of people covered by RML: This variable was derived by dividing the population in all states with RML with the total US population. The estimated proportion ranged from 4% in 2012 to 21% in 2016. These two variables were used to quantitatively assess the relationships between marijuana laws and changes in the risk of marijuana use.

Covariates: Demographic variables gender (male/female) and race/ethnicity (White, Black, Hispanic and others) were used to describe the study sample.

Statistical analysis

We estimated the prevalence of current marijuana use by year using the survey estimation method, considering the complex multi-stage cluster random sampling design and unequal probability. A prevalence rate is not a simple indicator, but consisting of the impact of chronological age, time period and birth cohort, named as age, period and cohort effects, respectively. Thus, it is biased if a prevalence rate is directly used to depict the historical trend. HAPC modeling is an epidemiological method capable of decomposing prevalence rate into mutually independent age, period and cohort effects with individual-level data, while the estimated period and cohort effects provide an unbiased measure of historical trend controlling for the effects of age and other covariates. In this study, we analyzed the data using the two-level HAPC cross-classified random-effects model (CCREM) [ 36 ]:

Where M ijk represents the rate of marijuana use for participants in age group i (12–13, 14,15...), period j (1979, 1982,...) and birth cohort k (1954–55, 1956–57...); parameter α i (age effect) was modeled as the fixed effect; and parameters β j (period effect) and γ k (cohort effect) were modeled as random effects; and β m was used to control m covariates, including the two dummy variables assessing changes made to the NSDUH in 1999 and 2002, respectively.

The HAPC modeling analysis was executed using the PROC GLIMMIX. Sample weights were included to obtain results representing the total US population aged 12–25. A ridge-stabilized Newton-Raphson algorithm was used for parameter estimation. Modeling analysis was conducted for the overall sample, stratified by gender. The estimated age effect α i , period β j and cohort γ k (i.e., the log-linear regression coefficients) were directly plotted to visualize the pattern of change.

To gain insight into the relationship between legal events and regulations at the national level, we listed these events/regulations along with the estimated time trends in the risk of marijuana from HAPC modeling. To provide a quantitative measure, we associated the estimated period effect with the proportions of US population living with MML and RML using Pearson correlation. All statistical analyses for this study were conducted using the software SAS, version 9.4 (SAS Institute Inc., Cary, NC).

Sample characteristics

Data for a total of 749,152 participants (12–25 years old) from all 31-wave NSDUH covering a 38-year period were analyzed. Among the total sample (Table  1 ), 48.96% were male and 58.78% were White, 14.84% Black, and 18.40% Hispanic.

Prevalence rate of current marijuana use

As shown in Fig.  1 , the estimated prevalence rates of current marijuana use from 1979 to 2016 show a “V” shaped pattern. The rate was 27.57% in 1979, it declined to 8.02% in 1992, followed by a gradual increase to 14.70% by 2016. The pattern was the same for both male and female with males more likely to use than females during the whole period.

Prevalence rate (%) of current marijuana use among US residents 12 to 25 years of age during 1979–2016, overall and stratified by gender. Derived from data from the 1979–2016 National Survey on Drug Use and Health (NSDUH)

HAPC modeling and results

Estimated age effects α i from the CCREM [ 1 ] for current marijuana use are presented in Fig.  2 . The risk by age shows a 2-phase pattern –a rapid increase phase from ages 12 to 19, followed by a gradually declining phase. The pattern was persistent for the overall sample and for both male and female subsamples.

Age effect for the risk of current marijuana use, overall and stratified by male and female, estimated with hierarchical age-period-cohort modeling method with 31 waves of NSDUH data during 1979–2016. Age effect α i were log-linear regression coefficients estimated using CCREM (1), see text for more details

The estimated period effects β j from the CCREM [ 1 ] are presented in Fig.  3 . The period effect reflects the risk of current marijuana use due to significant events occurring over the period, particularly federal and state laws and regulations. After controlling for the impacts of age, cohort and other covariates, the estimated period effect indicates that the risk of current marijuana use had two declining trends (1979–1992 and 2001–2006), and two increasing trends (1992–2001 and 2006–2016). Epidemiologically, the time trends characterized by the estimated period effects in Fig. 3 are more valid than the prevalence rates presented in Fig. 1 because the former was adjusted for confounders while the later was not.

Period effect for the risk of marijuana use for US adolescents and young adults, overall and by male/female estimated with hierarchical age-period-cohort modeling method and its correlation with the proportion of US population covered by Medical Marijuana Laws and Recreational Marijuana Laws. Period effect β j were log-linear regression coefficients estimated using CCREM (1), see text for more details

Correlation of the period effect with proportions of the population covered by marijuana laws: The Pearson correlation coefficient of the period effect with the proportions of US population covered by MML during 1996–2016 was 0.89 for the total sample, 0.81 for male and 0.93 for female, respectively ( p  < 0.01 for all). The correlation between period effect and proportion of US population covered by RML was 0.64 for the total sample, 0.59 for male and 0.49 for female ( p  > 0.05 for all).

Likewise, the estimated cohort effects γ k from the CCREM [ 1 ] are presented in Fig.  4 . The cohort effect reflects changes in the risk of current marijuana use over the period indicated by the year of birth of the survey participants after the impacts of age, period and other covariates are adjusted. Results in the figure show three distinctive cohorts with different risk patterns of current marijuana use during 1954–2003: (1) the Historical Declining Cohort (HDC): those born in 1954–1972, and characterized by a gradual and linear declining trend with some fluctuations; (2) the Sudden Increase Cohort (SIC): those born from 1972 to 1984, characterized with a rapid almost linear increasing trend; and (3) the Contemporary Declining Cohort (CDC): those born during 1984 and 2003, and characterized with a progressive declining over time. The detailed results of HAPC modeling analysis were also shown in Additional file 1 : Table S1.

Cohort effect for the risk of marijuana use among US adolescents and young adults born during 1954–2003, overall and by male/female, estimated with hierarchical age-period-cohort modeling method. Cohort effect γ k were log-linear regression coefficients estimated using CCREM (1), see text for more details

This study provides new data regarding the risk of marijuana use in youth in the US during 1954–2016. This is a period in the US history with substantial increases and declines in drug use, including marijuana; accompanied with many ups and downs in legal actions against drug use since the 1970s and progressive marijuana legalization at the state level from the later 1990s till today (see Additional file 1 : Table S2). Findings of the study indicate four-phase period effect and three-phase cohort effect, corresponding to various historical events of marijuana laws, regulations and social movements.

Coincident relationship between the period effect and legal drug control

The period effect derived from the HAPC model provides a net effect of the impact of time on marijuana use after the impact of age and birth cohort were adjusted. Findings in this study indicate that there was a progressive decline in the period effect during 1979 and 1992. This trend was corresponding to a period with the strongest legal actions at the national level, the War on Drugs by President Nixon (1969–1974) President Reagan (1981–1989) [ 45 ], and President Bush (1989) [ 45 ],and the Anti-Drug Abuse Act (1986) [ 5 ].

The estimated period effect shows an increasing trend in 1992–2001. During this period, President Clinton advocated for the use of treatment to replace incarceration (1992) [ 45 ], Surgeon General Elders proposed to study marijuana legalization (1993–1994) [ 8 ], President Clinton’s position of the need to re-examine the entire policy against people who use drugs, and decriminalization of marijuana (2000) [ 45 ] and the passage of MML in eight US states.

The estimated period effect shows a declining trend in 2001–2006. Important laws/regulations include the Student Drug Testing Program promoted by President Bush, and the broadened the public schools’ authority to test illegal drugs among students given by the US Supreme Court (2002) [ 46 ].

The estimated period effect increases in 2006–2016. This is the period when the proportion of the population covered by MML progressively increased. This relation was further proved by a positive correlation between the estimated period effect and the proportion of the population covered by MML. In addition, several other events occurred. For example, over 500 economists wrote an open letter to President Bush, Congress and Governors of the US and called for marijuana legalization (2005) [ 47 ], and President Obama ended the federal interference with the state MML, treated marijuana as public health issues, and avoided using the term of “War on Drugs” [ 45 ]. The study also indicates that the proportion of population covered by RML was positively associated with the period effect although not significant which may be due to the limited number of data points of RML. Future studies may follow up to investigate the relationship between RML and rate of marijuana use.

Coincident relationship between the cohort effect and legal drug control

Cohort effect is the risk of marijuana use associated with the specific year of birth. People born in different years are exposed to different laws, regulations in the past, therefore, the risk of marijuana use for people may differ when they enter adolescence and adulthood. Findings in this study indicate three distinctive cohorts: HDC (1954–1972), SIC (1972–1984) and CDC (1984–2003). During HDC, the overall level of marijuana use was declining. Various laws/regulations of drug use in general and marijuana in particular may explain the declining trend. First, multiple laws passed to regulate the marijuana and other substance use before and during this period remained in effect, for example, the Marijuana Tax Act (1937), the Boggs Act (1952), the Narcotics Control Act (1956) and the Controlled Substance Act (1970). Secondly, the formation of government departments focusing on drug use prevention and control may contribute to the cohort effect, such as the Bureau of Narcotics and Dangerous Drugs (1968) [ 48 ]. People born during this period may be exposed to the macro environment with laws and regulations against marijuana, thus, they may be less likely to use marijuana.

Compared to people born before 1972, the cohort effect for participants born during 1972 and 1984 was in coincidence with the increased risk of using marijuana shown as SIC. This trend was accompanied by the state and federal movements for marijuana use, which may alter the social environment and public attitudes and beliefs from prohibitive to acceptive. For example, seven states passed laws to decriminalize the marijuana use and reduced the penalty for personal possession of small amount of marijuana in 1976 [ 7 ]. Four more states joined the movement in two subsequent years [ 7 ]. People born during this period may have experienced tolerated environment of marijuana, and they may become more acceptable of marijuana use, increasing their likelihood of using marijuana.

A declining cohort CDC appeared immediately after 1984 and extended to 2003. This declining cohort effect was corresponding to a number of laws, regulations and movements prohibiting drug use. Typical examples included the War on Drugs initiated by President Nixon (1980s), the expansion of the drug war by President Reagan (1980s), the highly-publicized anti-drug campaign “Just Say No” by First Lady Nancy Reagan (early 1980s) [ 45 ], and the Zero Tolerance Policies in mid-to-late 1980s [ 45 ], the Anti-Drug Abuse Act (1986) [ 5 ], the nationally televised speech of War on Drugs declared by President Bush in 1989 and the escalated War on Drugs by President Clinton (1993–2001) [ 45 ]. Meanwhile many activities of the federal government and social groups may also influence the social environment of using marijuana. For example, the Federal government opposed to legalize the cultivation of industrial hemp, and Federal agents shut down marijuana sales club in San Francisco in 1998 [ 48 ]. Individuals born in these years grew up in an environment against marijuana use which may decrease their likelihood of using marijuana when they enter adolescence and young adulthood.

This study applied the age-period-cohort model to investigate the independent age, period and cohort effects, and indicated that the model derived trends in marijuana use among adolescents and young adults were coincident with the laws and regulations on marijuana use in the United States since the 1950s. With more states legalizing marijuana use in the United States, emphasizing responsible use would be essential to protect youth from using marijuana.

Limitations

This study has limitations. First, study data were collected through a household survey, which is subject to underreporting. Second, no causal relationship can be warranted using cross-sectional data, and further studies are needed to verify the association between the specific laws/regulation and the risk of marijuana use. Third, data were available to measure single-year age up to age 21 and two-year age group up to 25, preventing researchers from examining the risk of marijuana use for participants in other ages. Lastly, data derived from NSDUH were nation-wide, and future studies are needed to analyze state-level data and investigate the between-state differences. Although a systematic review of all laws and regulations related to marijuana and other drugs is beyond the scope of this study, findings from our study provide new data from a historical perspective much needed for the current trend in marijuana legalization across the nation to get the benefit from marijuana while to protect vulnerable children and youth in the US. It provides an opportunity for stack-holders to make public decisions by reviewing the findings of this analysis together with the laws and regulations at the federal and state levels over a long period since the 1950s.

Availability of data and materials

The data of the study are available from the designated repository ( https://nsduhweb.rti.org/respweb/homepage.cfm ).

Abbreviations

Audio computer-assisted self-interviews

Age-period-cohort modeling

Computer-assisted person interviews

Cross-classified random-effects model

Contemporary Declining Cohort

Hierarchical age-period-cohort

Historical Declining Cohort

Medical Marijuana Laws

National Household Survey on Drug Abuse

National Survey on Drug Use and Health

Recreational Marijuana Laws

Sudden Increase Cohort

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Additional file 1: table s1..

Estimated Age, Period, Cohort Effects for the Trend of Marijuana Use in Past Month among Adolescents and Emerging Adults Aged 12 to 25 Years, NSDUH, 1979-2016. Table S2. Laws at the federal and state levels related to marijuana use.

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Yu, B., Chen, X., Chen, X. et al. Marijuana legalization and historical trends in marijuana use among US residents aged 12–25: results from the 1979–2016 National Survey on drug use and health. BMC Public Health 20 , 156 (2020). https://doi.org/10.1186/s12889-020-8253-4

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The Public Health Effects of Legalizing Marijuana

Thirty-six states have legalized medical marijuana and 14 states have legalized the use of marijuana for recreational purposes. In this paper, we review the literature on the public health consequences of legalizing marijuana, focusing on studies that have appeared in economics journals as well as leading public policy, public health, and medical journals. Among the outcomes considered are: youth marijuana use, alcohol consumption, the abuse of prescription opioids, traffic fatalities, and crime. For some of these outcomes, there is a near consensus in the literature regarding the effects of medical marijuana laws (MMLs). As an example, leveraging geographic and temporal variation in MMLs, researchers have produced little credible evidence to suggest that legalization promotes marijuana use among teenagers. Likewise, there is convincing evidence that young adults consume less alcohol when medical marijuana is legalized. For other public health outcomes such as mortality involving prescription opioids, the effect of legalizing medical marijuana has proven more difficult to gauge and, as a consequence, we are less comfortable drawing firm conclusions. Finally, it is not yet clear how legalizing marijuana for recreational purposes will affect these and other important public health outcomes. We will be able to draw stronger conclusions when more post-treatment data are collected in states that have recently legalized recreational marijuana.

The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.

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D. Mark Anderson & Daniel I. Rees, 2023. " The Public Health Effects of Legalizing Marijuana, " Journal of Economic Literature, vol 61(1), pages 86-143.

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Estimated coefficients are shown for the overall population, adults, and youths; error bars indicate 95% CIs. The year before implementation was the reference (omitted) year. The estimated coefficient should be interpreted as relative to this year. The final lag or lead points accumulated all years beyond (ie, −4 included year −4 and earlier; 3 included year 3 and later). All regressions also included controls for the presence of medical cannabis legalization, share of the population with less than a high school diploma or equivalent, share of female individuals in the population, share of individuals in the population from racial and ethnic minority groups, share of youths in the population, number of police officers per 1000 population, unemployment rate, income per capita in 2019 thousand dollars, poverty rate, state and year indicators, and state-specific time trends. Standard errors were clustered at the state level. All regressions were weighted by state population averaged over the study period (2010-2019).

Estimated coefficients are shown for the overall population, adults, and youths by racial group; error bars indicate 95% CIs. The year before implementation was the reference (omitted) year. The estimated coefficient should be interpreted as relative to this year. The final lag or lead points accumulated all years beyond (ie, −4 included year −4 and earlier; 3 included year 3 and later). All regressions also included controls for the presence of medical cannabis legalization, share of the population with less than a high school diploma or equivalent, share of female individuals in the population, share of individuals in the population from racial and ethnic minority groups, share of youths in the population, number of police officers per 1000 population, unemployment rate, income per capita in 2019 thousand dollars, poverty rate, state and year indicators, and state-specific time trends. Standard errors were clustered at the state level. All regressions were weighted by state population averaged over the study period (2010-2019).

eTable 1. States Included and Excluded in the Study

eFigure 1. Trends in Overall Cannabis Possession Arrest Rates in Recreational Cannabis Legalization States

eFigure 2. Trends in Adult Cannabis Possession Arrest Rates in Recreational Cannabis Legalization States

eFigure 3. Trends in Youth Cannabis Possession Arrest Rates in Recreational Cannabis Legalization States

eTable 2. Regression Results for the Overall Population: Recreational Cannabis Legalization States Without Cannabis Decriminalization Already in Place

eTable 3. Regression Results for Adults: Recreational Cannabis Legalization States Without Cannabis Decriminalization Already in Place

eTable 4. Regression Results for Youths: Recreational Cannabis Legalization States Without Cannabis Decriminalization Already in Place

eTable 5. Regression Results for the Overall Population: Recreational Cannabis Legalization States With Cannabis Decriminalization Already in Place

eTable 6. Regression Results for Adults: Recreational Cannabis Legalization States With Cannabis Decriminalization Already in Place

eTable 7. Regression Results for Youths: Recreational Cannabis Legalization States With Cannabis Decriminalization Already in Place

eFigure 4. Leave-One-Out Analysis for Recreational Cannabis Legalization States Without Decriminalization Already in Place

eFigure 5. Leave-One-Out Analysis for Recreational Cannabis Legalization States With Decriminalization Already in Place

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Gunadi C , Shi Y. Association of Recreational Cannabis Legalization With Cannabis Possession Arrest Rates in the US. JAMA Netw Open. 2022;5(12):e2244922. doi:10.1001/jamanetworkopen.2022.44922

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Association of Recreational Cannabis Legalization With Cannabis Possession Arrest Rates in the US

• 1 Herbert Wertheim School of Public Health and Human Longevity Science, University of California San Diego, La Jolla

Question   Is recreational cannabis legalization (RCL) associated with a reduction in cannabis possession arrest rates in US states that have already decriminalized cannabis; if so, does this change differ by age and race?

Findings   In this cross-sectional study using difference-in-differences analysis for data from 31 states, RCL was associated with a 40% reduction in cannabis possession arrest rates among adults in the 5 states that had already decriminalized cannabis between 2010 and 2019. This decrease was significantly smaller than that for the 4 states that had not decriminalized cannabis (76%), and there was no association between RCL and changes in youth arrest rates or disparities in arrest rates among Black and White individuals.

Meaning   These findings suggest that RCL was associated with decreased cannabis possession arrest rates among adults during the study period, even in US states that had already decriminalized cannabis.

Importance   Recreational cannabis legalization (RCL) has been advocated as a way to reduce the number of individuals interacting with the US criminal justice system; in theory, however, cannabis decriminalization can achieve this objective without generating the negative public health consequences associated with RCL. It is still unclear whether RCL can bring additional benefits in terms of reducing cannabis possession arrests in states that have already decriminalized cannabis.

Objective   To examine whether RCL was associated with changes in cannabis possession arrests in US states that had already decriminalized cannabis during the study period and whether these changes differed across age and racial subgroups.

Design, Setting, and Participants   This repeated cross-sectional study used cannabis possession arrest data from the Federal Bureau of Investigation Uniform Crime Reporting Program (UCRP) for US states from 2010 through 2019. Statistical analysis was conducted from October 6, 2021, to October 12, 2022.

Exposures   Implementation of statewide RCL.

Main Outcomes and Measures   Cannabis possession arrest rates per 1000 population per year were assessed with a quasi-experimental difference-in-differences design and were used to estimate the association of RCL with arrest rates in RCL states that had or had not decriminalized cannabis before RCL. This association was also examined in subgroups for age (adults vs youths) and race (Black vs White).

Results   This study included UCRP data for 31 US states, including 9 states that implemented RCL during the study period (4 without and 5 with decriminalization) and 22 non-RCL states. In the 4 states that had not decriminalized cannabis before legalization, RCL was associated with a 76.3% decrease (95% CI, −81.2% to −69.9%) in arrest rates among adults. In the 5 states that had already decriminalized cannabis, RCL was still associated with a substantial decrease in adult arrest rates (−40.0%; 95% CI, −55.1% to −19.8%). There was no association of RCL with changes in arrest rates among youths. In addition, changes in arrest rates associated with RCL did not differ among Black and White individuals.

Conclusions and Relevance   In this repeated cross-sectional study, RCL was associated with a sizable reduction in cannabis possession arrests among adults in states that had already decriminalized cannabis during the study period (2010-2019), albeit the magnitude was smaller compared with states that had not decriminalized cannabis before RCL. In addition, RCL did not seem to be associated with changes in arrest rates among youths or disparities in arrest rates among Black and White individuals.

In 2012, Colorado and Washington became the first US states to implement recreational cannabis legalization (RCL), which allows the possession of a small amount of cannabis for adult use without any penalties. Since then, 16 states and the District of Columbia have followed suit. 1 One reason for this momentum is the increasing recognition that cannabis illegality sends a large number of individuals to the criminal justice system, with adverse physical, mental, and social consequences. 2

To reduce the number of cannabis possession arrests, civil liberties advocates have voiced support for RCL. 2 In theory, however, a reduction in cannabis possession arrests can be achieved by decriminalizing cannabis, which changes the penalties associated with a small amount of cannabis possession from criminal to civil infractions. In fact, more than 30 states have adopted cannabis decriminalization since the 1970s. The latest research estimates that during the 2010s, cannabis possession arrest rates were reduced substantially among both adults and youths (by 40%-80%) as a result of decriminalization. 3 - 5 Cannabis decriminalization and RCL have substantial differences in terms of motivations, enactment, provisions, and enforcements. Both, however, have the potential to reduce cannabis possession arrests. If the policy goal is solely to reduce arrests, decriminalization may seem to be more appealing than RCL. First, under cannabis decriminalization, individuals caught possessing a small amount of cannabis are still subject to civil penalties (eg, fines, mandated drug education programs, or public services) but do not receive criminal sanctions. The civil penalties may serve as a deterrent to cannabis use and prevent related adverse consequences. Second, RCL could allow the legal sale of cannabis, which increases access and exposure to cannabis products and marketing activities. The legality of cannabis may also make social norms more favorable to cannabis. Some studies have shown that RCL is associated with increased use of cannabis among both adults and youths, creating considerable public health concerns. 6 - 9

In contrast, RCL has potential to further reduce cannabis possession arrests on top of decriminalization. Under RCL, law enforcement agents are no longer required to look for cannabis possession violations in a small amount. Although possession of a large amount of cannabis is still a criminal offense under both decriminalization and RCL, an overall lower police search rate under RCL will likely lead to a large reduction in arrests even in states that have already decriminalized cannabis.

We are aware of only 1 study that has examined the association of RCL with cannabis possession arrests. 5 In a comparison of 4 US states that implemented RCL from 2000 to 2016 with other states that did not change penalties for cannabis possession in the same period, Plunk et al 5 found that RCL was associated with a substantial decline in arrest rates among adults but not youths. The authors did not distinguish RCL states that had already decriminalized cannabis from those that had not. 5 It remains unclear whether RCL can further reduce cannabis possession arrests in states that have already decriminalized cannabis.

This study had 2 objectives. First, we examined whether RCL implementation was associated with a reduction in cannabis possession arrests in states that had already decriminalized cannabis. If no evidence was found, it might be plausible for policy makers in states already implementing decriminalization to consider alternative strategies other than RCL to further reduce arrests. We also examined whether the association existed in states without cannabis decriminalization. If no evidence was found, decriminalization might be considered a more effective and safer strategy of reducing arrests without the potential negative consequences associated with RCL.

Second, we assessed whether the association of RCL with cannabis possession arrests differed by age (adults vs youths) and race (Black vs White). Recreational cannabis has been legalized only for adult use, which in theory should have minimal effects on arrests of youths. According to a previous American Civil Liberties Union report, Black individuals were considerably more likely to be arrested for cannabis possession compared with White individuals, despite a similar rate of cannabis use. 10 The magnitude of the change in arrests after RCL could therefore differ among Black and White individuals.

This cross-sectional study used publicly available secondary data sources and was therefore deemed exempt from institutional review board approval and informed consent per University of California San Diego policy. The study followed the Strengthening the Reporting of Observational Studies in Epidemiology ( STROBE ) reporting guideline.

Data on cannabis possession arrests by age, sex, and race were obtained from the Uniform Crime Reporting Program (UCRP) for 2010 to 2019. 11 The UCRP is assembled from more than 18 000 law enforcement agencies and maintained by the US Federal Bureau of Investigation, and it is one of the most widely used data sources to examine crimes in the US. 4 , 5 , 12

UCRP arrest data have limitations. First, reporting to the UCRP by law enforcement agencies is voluntary; hence, some states in some periods may have incomplete data. In particular, Florida did not report arrest statistics for most years (2010-2016) in the study period; we thus excluded Florida from this study. Second, there was a misreporting in the number of arrests by the Denver Police Department after cannabis was legalized in Colorado. 5 Because this department is one of the largest police divisions in Colorado, misreporting in Denver could result in incorrect statistics for the entire state; we consequently excluded Colorado from this study. Third, information on arrests by race and ethnicity was limited. Race information was available only for American Indian or Alaska Native, Asian, Black, and White individuals. Data on American Indian or Alaska Native and Asian individuals were not assessed in this study because cannabis possession arrests among these groups are rare and the disparity in arrest rates is most striking between Black and White individuals. Ethnicity information, such as Hispanic origin, was unavailable for most of the study period. Race and ethnicity information is often reported by officers and subject to inaccuracy. Finally, the UCRP adheres to the hierarchy rule in its reporting, such that less severe crimes (part II offenses) like cannabis possession are reported only if they occur during an incident without the more severe crimes (part I offenses). 13

Our main outcome was annual cannabis possession arrest rates per 1000 population at the state level. The state-level population estimates were obtained from the Integrated Public Use Microdata Series American Community Survey (ACS) for 2010 to 2019. 14 The rates were calculated separately in adult and youth populations and separately for Black and White individuals. Although the legal purchase age for recreational cannabis was 21 years in all RCL states, we used age 18 years as the cutoff to define adults and youths in this study because the UCRP reports arrests by race in only 2 age categories based on this cutoff. 11 Eighteen years was also used as the cutoff age in previous literature. 4 , 5 , 12

The state-level policy exposure was a binary indicator taking the value of 1 if RCL was in effect in a year and a state and 0 otherwise.

There were 9 states that implemented RCL in the study period (eTable 1 in Supplement 1 ). Two groups of RCL states were constructed as follows based on whether cannabis decriminalization was already in place before RCL implementation 4 , 5 , 15 - 17 : (1) RCL states without decriminalization already in place, including Washington, Alaska, Nevada, and Michigan; and (2) RCL states with decriminalization already in place, including Oregon, California, Massachusetts, Maine, and Vermont. Because California and Vermont decriminalized cannabis during the study period, we excluded their state–year observations before and in the year of decriminalization. These 2 groups of RCL states were analyzed separately to allow differential effects of RCL by decriminalization status.

The comparison group consisted of 22 states that neither implemented RCL nor changed penalties for cannabis possession in the study period (ie, non-RCL states). 4 , 5 , 15 - 17 Seventeen states that did not implement RCL were excluded from the study because they either decriminalized cannabis or changed penalties for cannabis possession during the study period. 2 , 4 , 5 eTable 1 in Supplement 1 lists all included and excluded states.

Following previous studies, time-varying state-level covariates that may confound the association of RCL with cannabis possession arrests were included in the regressions. 3 - 5 These covariates included the share of the population with less than a high school diploma or equivalent, share of female individuals in the population, share of individuals in the population from racial and ethnic minority groups, share of youths in the population, number of police officers per 1000 population, unemployment rate, income per capita in 2019 thousand dollars, and poverty rate. These state-level characteristics were constructed from ACS and UCRP data on law enforcement officers killed and assaulted. 18 The binary indicator for the presence of medical cannabis legalization was also included in the regressions, taking the value of 1 if it was in effect in a year and a state and 0 otherwise. 4 , 5 The effective dates of medical cannabis legalization were obtained from ProCon.org.

The unit of analysis was state–year observations. To examine the association of RCL with cannabis possession arrest rates, we used the quasi-experimental difference-in-differences research design. Specifically, we used log-linear regression to model arrest rates as a function of RCL, adjusting for all covariates mentioned in the previous section, fixed effects for state and year, and state-specific time trends. State fixed effects (indicators of states) accounted for potential state-specific confounding factors that did not vary over time. Year fixed effects (indicators of years) accounted for secular trends in outcomes common to all states in the same year. State-specific time trends (linear trend variable for each state) accounted for state-specific confounding factors that varied linearly over time. Two types of RCL states (with and without cannabis decriminalization) were compared with comparison states separately. The analysis was also conducted separately for adults and youths and for Black and White individuals.

Vermont reported 0 cannabis possession arrests for Black adults in 2015 and 2019. To avoid dropping these state–year observations, we added a small constant (0.01) to all state–year observations for log transformation in all race-related analysis. Our results were not sensitive to this specification.

As noted earlier, the voluntary reporting to UCRP potentially exerts bias in the estimated association due to measurement error. Because this error was more severe in areas with a small population size, we followed previous studies to weight the regression by state population size averaged over the entire study period to minimize potential bias. 19 , 20 Standard errors were clustered at the state level to account for possible serial correlation within a state. 21

We formally tested differences in the effects of RCL among (1) RCL states with and without decriminalization already in place, (2) adults and youths, and (3) Black and White individuals. Specifically, we used seemingly unrelated estimation and tested the equality between regression coefficients in the comparison.

We also conducted 2 supplemental analyses. First, we conducted an event study by replacing the RCL indicator with a series of its leads and lags to examine whether the estimated association was driven by the difference in prelegalization trends between RCL and non-RCL states. Second, we conducted a leave-one-out exercise by dropping 1 RCL state from the regression at a time to examine whether the overall estimate was driven by a specific RCL state. P  < .05 was considered statistically significant. Statistical analyses were performed with Stata, version 16 (StataCorp LLC).

Table 1 reports outcome and covariate statistical results by state RCL and decriminalization status. eFigure 1 in Supplement 1 plots trends in cannabis possession arrest rates in RCL states during the study period. We observed a substantial decrease in arrest rates after RCL was implemented in states that had already decriminalized cannabis, although this decline seemed smaller compared with RCL states that had not decriminalized cannabis. The magnitude of the decline seemed to be similar between Black and White individuals regardless of state decriminalization status. The decrease in arrest rate seemed to be driven by adults but not youths (eFigures 2 and 3 in Supplement 1 , respectively).

Table 2 reports association estimation results from regressions (detailed results in eTables 2-7 in Supplement 1 ). Table 3 reports the test statistics for the differences in association coefficients among (1) RCL states with and without cannabis decriminalization, (2) adults and youths, and (3) Black and White individuals.

Recreational cannabis legalization was associated with a 68.6% decline (95% CI, −75.6% to −59.8%) in arrest rates in RCL states that had not decriminalized cannabis before RCL and a 33.0% decline (95% CI, −49.3% to −11.3%) in RCL states that had already decriminalized cannabis. The association difference in the 2 types of RCL states was statistically significant ( P  < .001).

Among adults, RCL was associated with a 76.3% decline (95% CI, −81.2% to −69.9%) in arrest rates in RCL states that had not decriminalized cannabis before RCL and a 40.0% decline (95% CI, −55.1% to −19.8%) in RCL states that had already decriminalized cannabis. Among youths, RCL was not associated with a change in arrest rate.

In RCL states that had not decriminalized cannabis before legalization, RCL was associated with a 77.9% decline (95% CI, −84.3% to −69.3%) in arrest rates among Black adults and a 74.6% decline (95% CI, −79.0% to −69.3%) among White adults. In RCL states that had already decriminalized cannabis before legalization, RCL was associated with a 35.0% decline (95% CI, −55.5% to −5.5%) in arrest rates among Black adults and a 41.1% decline (95% CI, −55.1% to −23.7%) among White adults. However, there was no evidence that the decreases differed between Black and White adults in either type of RCL state. There was no association between RCL and arrest rate in Black or White youths.

Figure 1 and Figure 2 report results from the event study. It appeared that the estimated associations were not driven by differences in prelegalization trends between RCL and non-RCL states. The validity of the difference-in-differences design was hence supported. The association might be dynamic: in most cases, the decline seemed to be larger in magnitude after the year of RCL implementation. The dynamic association should be interpreted with caution, however, because some states had very limited time points in the post-RCL period.

eFigures 4 and 5 in Supplement 1 report results from the leave-one-out analysis. Overall, it seems that the main findings were not driven by a specific RCL state.

This cross-sectional study examined the association of RCL with changes in cannabis possession arrests and tested whether the association differed between RCL states with and without decriminalization already in place. Our results suggest that RCL was associated with a substantial decrease in adult arrest rates in both types of RCL states for the study period (2010-2019). For adults, the magnitude of the decrease (40.0%-76.3%) associated with RCL was comparable to that associated with cannabis decriminalization. 3 - 5 States that had already decriminalized cannabis before RCL saw a smaller magnitude of decline (40.0%) than states that had not decriminalized cannabis before RCL (76.3%). These findings suggest that implementing RCL may be associated with a further reduction in adult arrest rates even after a state decriminalizes cannabis.

Consistent with a 2019 study by Plunk et al, 5 we did not find an association between RCL and cannabis possession arrests among youths regardless of decriminalization status in RCL states. This finding was not surprising because RCL intends to legalize cannabis use among adults but not youths. If youth arrest is more concerning because of the prolonged, adverse health and socioeconomic consequences from adolescence to adulthood, 22 cannabis decriminalization may be a preferred strategy because it removes criminal penalties not only for adults but also youths and also reduces arrests in both age groups. 3 - 5

Despite similar rates of cannabis use, Black individuals are reportedly 3 to 4 times more likely to be arrested for cannabis possession compared with White individuals. 10 A previous study estimated that cannabis decriminalization was associated with an approximately 17.0% reduction in racial disparities in arrests among Black and White adults. 3 Our results suggest that RCL might not provide additional benefits in terms of reducing racial disparities compared with decriminalization. Nonetheless, we should note that the decrease in cannabis possession arrests after RCL was substantial for both Black and White adults, demonstrating an overall change in law enforcement behaviors.

If we compare the benefits of RCL and cannabis decriminalization based solely on their associations with cannabis possession arrests, this study and the existing literature suggest that both RCL and decriminalization are associated with a sizable reduction in adult arrest rates. 3 - 5 Even after decriminalization was implemented, adults could still benefit from a further reduction in arrests under RCL. The argument that RCL could reduce individual contact with the criminal justice system is supported. Nonetheless, decriminalization has additional benefits; for example, it was also associated with reductions in arrests among youths and in racial disparities among Black and White individuals. 3 That said, the choice of RCL and decriminalization approaches should be made with a holistic evaluation of all benefits and costs. The effect on the criminal justice system is a major consideration but should not be the only one. Other considerations could include effects on public health, 6 the economy, and society. Policy makers are encouraged to adopt a strategy only when the total benefits outweigh the total costs.

This cross-sectional study is not without limitations. First, police agency reporting to the UCRP is voluntary. To account for measurement errors, we followed previous studies to weight the regressions by population. 19 , 20 However, this approach may not have fully eliminated the bias. Second, cannabis possession arrests may be underestimated due to the hierarchical reporting in the UCRP. We believe that it should not bias the difference-in-differences estimates unless RCL implementation was associated with changes in the reporting method or the volume of severe crimes. Third, UCRP information on race may be inaccurate. Fourth, we were not able to examine other races or ethnicities due to data limitations. Fifth, time-varying unobserved confounding factors may not have been fully accounted for by the difference-in-differences design. Furthermore, findings from the 9 RCL states may not necessarily generalize to other US states or outside of the US setting. Finally, it may take time for the effects on law enforcement behaviors to fully materialize after RCL. The post-RCL period in this study might be too short to capture changes in youths and racial disparities. A re-examination with a longer post-RCL period is warranted.

The findings of this repeated cross-sectional study suggest that RCL was associated with a substantial decrease in adult arrests in US states that had already decriminalized cannabis, albeit of a smaller magnitude compared with RCL states that had not. RCL did not appear to be associated with changes in arrest rates among youths or disparities in arrest rates among Black and White individuals.

Accepted for Publication: October 18, 2022.

Published: December 5, 2022. doi:10.1001/jamanetworkopen.2022.44922

Open Access: This is an open access article distributed under the terms of the CC-BY License . © 2022 Gunadi C et al. JAMA Network Open .

Corresponding Author: Yuyan Shi, PhD, Herbert Wertheim School of Public Health and Human Longevity Science, University of California San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0628 ( [email protected] ).

Author Contributions: Drs Gunadi and Shi had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Shi.

Acquisition, analysis, or interpretation of data: Both authors.

Drafting of the manuscript: Both authors.

Critical revision of the manuscript for important intellectual content: Shi.

Statistical analysis: Gunadi.

Obtained funding: Shi.

Administrative, technical, or material support: Shi.

Supervision: Shi.

Conflict of Interest Disclosures: Dr Gunadi reported receiving grants from the National Institute on Drug Abuse during the conduct of the study. Dr Shi reported receiving grants from the National Institute on Drug Abuse during the conduct of the study.

Funding/Support: This work was supported by award numbers R01DA042290 and R01DA049730 from the National Institute on Drug Abuse (Dr Shi).

Role of the Funder/Sponsor: The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Data Sharing Statement: See Supplement 2 .

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The public health effects of legalizing marijuana.

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The Evidence—and Lack Thereof—About Cannabis

Research is still needed on cannabis’s risks and benefits. 

Lindsay Smith Rogers

Although the use and possession of cannabis is illegal under federal law, medicinal and recreational cannabis use has become increasingly widespread.

Thirty-eight states and Washington, D.C., have legalized medical cannabis, while 23 states and D.C. have legalized recreational use. Cannabis legalization has benefits, such as removing the product from the illegal market so it can be taxed and regulated, but science is still trying to catch up as social norms evolve and different products become available. 

In this Q&A, adapted from the August 25 episode of Public Health On Call , Lindsay Smith Rogers talks with Johannes Thrul, PhD, MS , associate professor of Mental Health , about cannabis as medicine, potential risks involved with its use, and what research is showing about its safety and efficacy. 

Do you think medicinal cannabis paved the way for legalization of recreational use?

The momentum has been clear for a few years now. California was the first to legalize it for medical reasons [in 1996]. Washington and Colorado were the first states to legalize recreational use back in 2012. You see one state after another changing their laws, and over time, you see a change in social norms. It's clear from the national surveys that people are becoming more and more in favor of cannabis legalization. That started with medical use, and has now continued into recreational use.

But there is a murky differentiation between medical and recreational cannabis. I think a lot of people are using cannabis to self-medicate. It's not like a medication you get prescribed for a very narrow symptom or a specific disease. Anyone with a medical cannabis prescription, or who meets the age limit for recreational cannabis, can purchase it. Then what they use it for is really all over the place—maybe because it makes them feel good, or because it helps them deal with certain symptoms, diseases, and disorders.

Does cannabis have viable medicinal uses?

The evidence is mixed at this point. There hasn’t been a lot of funding going into testing cannabis in a rigorous way. There is more evidence for certain indications than for others, like CBD for seizures—one of the first indications that cannabis was approved for. And THC has been used effectively for things like nausea and appetite for people with cancer.

There are other indications where the evidence is a lot more mixed. For example, pain—one of the main reasons that people report for using cannabis. When we talk to patients, they say cannabis improved their quality of life. In the big studies that have been done so far, there are some indications from animal models that cannabis might help [with pain]. When we look at human studies, it's very much a mixed bag. 

And, when we say cannabis, in a way it's a misnomer because cannabis is so many things. We have different cannabinoids and different concentrations of different cannabinoids. The main cannabinoids that are being studied are THC and CBD, but there are dozens of other minor cannabinoids and terpenes in cannabis products, all of varying concentrations. And then you also have a lot of different routes of administration available. You can smoke, vape, take edibles, use tinctures and topicals. When you think about the explosion of all of the different combinations of different products and different routes of administration, it tells you how complicated it gets to study this in a rigorous way. You almost need a randomized trial for every single one of those and then for every single indication.

What do we know about the risks of marijuana use?  

Cannabis use disorder is a legitimate disorder in the DSM. There are, unfortunately, a lot of people who develop a problematic use of cannabis. We know there are risks for mental health consequences. The evidence is probably the strongest that if you have a family history of psychosis or schizophrenia, using cannabis early in adolescence is not the best idea. We know cannabis can trigger psychotic symptoms and potentially longer lasting problems with psychosis and schizophrenia. 

It is hard to study, because you also don't know if people are medicating early negative symptoms of schizophrenia. They wouldn't necessarily have a diagnosis yet, but maybe cannabis helps them to deal with negative symptoms, and then they develop psychosis. There is also some evidence that there could be something going on with the impact of cannabis on the developing brain that could prime you to be at greater risk of using other substances later down the road, or finding the use of other substances more reinforcing. 

What benefits do you see to legalization?

When we look at the public health landscape and the effect of legislation, in this case legalization, one of the big benefits is taking cannabis out of the underground illegal market. Taking cannabis out of that particular space is a great idea. You're taking it out of the illegal market and giving it to legitimate businesses where there is going to be oversight and testing of products, so you know what you're getting. And these products undergo quality control and are labeled. Those labels so far are a bit variable, but at least we're getting there. If you're picking up cannabis at the street corner, you have no idea what's in it. 

And we know that drug laws in general have been used to criminalize communities of color and minorities. Legalizing cannabis [can help] reduce the overpolicing of these populations.

What big questions about cannabis would you most like to see answered?

We know there are certain, most-often-mentioned conditions that people are already using medical cannabis for: pain, insomnia, anxiety, and PTSD. We really need to improve the evidence base for those. I think clinical trials for different cannabis products for those conditions are warranted.

Another question is, now that the states are getting more tax revenue from cannabis sales, what are they doing with that money? If you look at tobacco legislation, for example, certain states have required that those funds get used for research on those particular issues. To me, that would be a very good use of the tax revenue that is now coming in. We know, for example, that there’s a lot more tax revenue now that Maryland has legalized recreational use. Maryland could really step up here and help provide some of that evidence.

Are there studies looking into the risks you mentioned?

Large national studies are done every year or every other year to collect data, so we already have a pretty good sense of the prevalence of cannabis use disorder. Obviously, we'll keep tracking that to see if those numbers increase, for example, in states that are legalizing. But, you wouldn't necessarily expect to see an uptick in cannabis use disorder a month after legalization. The evidence from states that have legalized it has not demonstrated that we might all of a sudden see an increase in psychosis or in cannabis use disorder. This happens slowly over time with a change in social norms and availability, and potentially also with a change in marketing. And, with increasing use of an addictive substance, you will see over time a potential increase in problematic use and then also an increase in use disorder.

If you're interested in seeing if cannabis is right for you, is this something you can talk to your doctor about?

I think your mileage may vary there with how much your doctor is comfortable and knows about it. It's still relatively fringe. That will very much depend on who you talk to. But I think as providers and professionals, everybody needs to learn more about this, because patients are going to ask no matter what.

Lindsay Smith Rogers, MA, is the producer of the Public Health On Call podcast , an editor for Expert Insights , and the director of content strategy for the Johns Hopkins Bloomberg School of Public Health.

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• In Debate Over Legalizing Marijuana, Disagreement Over Drug’s Dangers

In Their Own Words: Supporters and Opponents of Legalization

Table of contents.

• About the Survey

Survey Report

Public opinion about legalizing marijuana, while little changed in the past few years, has undergone a dramatic long-term shift. A new survey finds that 53% favor the legal use of marijuana, while 44% are opposed.  As recently as 2006, just 32% supported marijuana legalization, while nearly twice as many (60%) were opposed.

Millennials (currently 18-34) have been in the forefront of this change: 68% favor legalizing marijuana use, by far the highest percentage of any age cohort. But across all generations –except for the Silent Generation (ages 70-87) – support for legalization has risen sharply over the past decade.

The latest national survey by the Pew Research Center, conducted March 25-29 among 1,500 adults, finds that supporters of legalizing the use of marijuana are far more likely than opponents to say they have changed their mind on this issue.

Among the public overall, 30% say they support legalizing marijuana use and have always felt that way, while 21% have changed their minds; they say there was a time when they thought it should be illegal. By contrast, 35% say they oppose legalization and have always felt that way; just 7% have changed their minds from supporting to opposing legalization.

When asked, in their own words, why they favor or oppose legalizing marijuana, people on opposite sides of the issue offer very different perspectives. But a common theme is the danger posed by marijuana: Supporters of legalization mention its perceived health benefits, or see it as no more dangerous than other drugs. To opponents, it is a dangerous drug, one that inflicts damage on people and society more generally.

The most frequently cited reasons for supporting the legalization of marijuana are its medicinal benefits (41%) and the belief that marijuana is no worse than other drugs (36%) –with many explicitly mentioning that they think it is no more dangerous than alcohol or cigarettes.

With four states and Washington, D.C. having passed measures to permit the use of marijuana for personal use, 27% of supporters say legalization would lead to improved regulation of marijuana and increased tax revenues. About one-in-ten (12%) cite the costs and problems of enforcing marijuana laws or say simply that people should be free to use marijuana (9%).

Why Should Marijuana Be Legal? Voices of Supporters

Main reason you support legalizing use of marijuana…

“My grandson was diagnosed with epilepsy a year ago and it has been proven that it helps with the seizures.” Female, 69

“I think crime would be lower if they legalized marijuana. It would put the drug dealers out of business.” Female, 62

“Because people should be allowed to have control over their body and not have the government intervene in that.” Male, 18

“I think that we would have more control over it by allowing a federal agency to tax and regulate it like alcohol.” Male, 25

The most frequently mentioned reason why people oppose legalization is that marijuana generally hurts society and is bad for individuals (43% say this). And while many supporters of legalization say that marijuana is less dangerous than other drugs, 30% of opponents have the opposite view: They point to the dangers of marijuana, including the possibility of abuse and addiction.

About one-in-five opponents of legalization (19%) say marijuana is illegal and needs to be policed, 11% say it is a gateway to harder drugs and 8% say it is especially harmful to young people. A small share of opponents (7%) say that while the recreational use of marijuana should be illegal, they do not object to legalizing medical marijuana. 1

Why Should Marijuana Be Illegal? Voices of Opponents

Main reason you oppose legalizing use of marijuana…

“It’s a drug and it has considerable side effects. It should not be used recreationally, only for medicinal use.” Female, 20

“It’s a drug that makes you stupid. It affects your judgment and motor skills and in the long term it makes you lazy.” Male, 52

“It gets too many people on drugs. It would put too many drugs on the street, we don’t need that.” Male, 84

“I’m thinking of my child. I don’t want her to try this. I know it’s not good for her health or brain.” Female, 33

“We have enough addictive things that are already legal. We don’t need another one.” Male, 42

Current Opinion on Legalizing Marijuana

The pattern of opinion about legalizing marijuana has changed little in recent years. Beyond the wide generation gap in support for legalization, there continue to be demographic and partisan differences.

Majorities of blacks (58%) and whites (55%) favor legalizing marijuana, compared with just 40% of Hispanics. Men (57% favor) continue to be more likely than women (49%) to support legalization.

Nearly six-in-ten Democrats (59%) favor legalizing the use of marijuana, as do 58% of independents. That compares with just 39% of Republicans.

Both parties are ideologically divided over legalizing marijuana. Conservative Republicans oppose legalizing marijuana by roughly two-to-one (65% to 32%); moderate and liberal Republicans are divided (49% favor legalization, 50% are opposed).

Among Democrats, 75% of liberals say the use of marijuana should be legal compared with half (50%) of conservative and moderate Democrats.

Other Opinions: Federal Enforcement of Marijuana Laws

The new survey also finds that as some states have legalized marijuana – placing them at odds with the federal prohibition against marijuana – a majority of Americans (59%) say that the federal government should not enforce laws in states that allow marijuana use; 37% say that they should enforce these laws. Views on federal enforcement of marijuana laws are unchanged since the question was first asked two years ago.

In contrast to overall attitudes about the legal use of marijuana, there are only modest differences in views across partisan groups: 64% of independents, 58% of Democrats and 54% of Republicans say that the federal government should not enforce federal marijuana laws in states that allow its use.

A substantial majority of those who say marijuana should be legal (78%) do not think the federal government should enforce federal laws in states that allow its use. Among those who think marijuana should be illegal, 59% say there should be federal enforcement in states that allow marijuana use, while 38% say there should not be.

Concerns About Marijuana Use

While most Americans support legalizing marijuana, there are concerns about public use of the drug, if it were to become legal. Overall, 62% say that if marijuana were legal it would bother them if people used it in public; just 33% say this would not bother them. Like overall views of legalizing marijuana, these views have changed little in recent years.

There is less concern about the possibility of a marijuana-related business opening legally in people’s own neighborhood: 57% say it would not bother them if a store or business selling marijuana opened legally in their neighborhood, while 41% say this would bother them.

And just 15% say they would be bothered if people used marijuana in their own homes; 82% say this would not bother them.

As might be expected, there are sharp differences in these concerns between people who favor and oppose legalizing marijuana. A large majority of opponents of marijuana legalization (85%) say they would be bothered by public use of the drug, if it were legal; about four-in-ten supporters (43%) also say they would be bothered by this. On the other hand, a majority of opponents of legalization (65%) say they would not be bothered if people used marijuana in their own homes; virtually all supporters of legalization (97%) would not be bothered by this.

And while 77% of those who oppose legalizing marijuana say, if it were legal, they would be bothered if a store or business selling marijuana opened in their neighborhood, just 12% of supporters of legalization say this would bother them.

About Half Say They Have Tried Marijuana

Overall, 49% say they have ever tried marijuana, while 51% say they have never done this. Self-reported experience with marijuana has shown no change over the past two years, but is higher than it was early last decade: In 2003, 38% said they had tried marijuana before, while 61% said they had not.

About a quarter of those who have tried marijuana (12% of the public overall) say they have used marijuana in the past year. Similar percentages reported using marijuana in the prior 12 months in two previous surveys, conducted in February 2014 and March 2013.

Men (56%) are 15 points more likely than women (41%) to say they have ever tried marijuana.

About half of whites (52%) and blacks (50%) say they have tried marijuana before. Among Hispanics, 36% say they have tried marijuana, while 63% say they have not.

Across generations, 59% of Baby Boomers say they’ve tried marijuana before; this compares with 47% of Generation Xers and 52% of Millennials. Among those in the Silent generation, only 19% say they have ever tried marijuana. Nearly a quarter of Millennials (23%) say they have used the drug in the past year, the highest share of any age cohort.

There is little difference in the shares of Democrats (48%) and Republicans (45%) who say they’ve tried marijuana. However, there are differences within each party by ideology. By a 61%-39% margin, most conservative Republicans say they have never tried marijuana. Among moderate and liberal Republicans, about as many say they have (52%) as have not (48%) tried marijuana before.

Among Democrats, liberals (58%) are more likely than conservatives and moderates (42%) to say they’ve tried marijuana.

While a majority of those who say marijuana should be legal say they’ve tried the drug before (65%), 34% of those who support legalization say they’ve never tried marijuana. Among those who say marijuana should be illegal, 29% say they have tried it before, while 71% say they have not.

• These are volunteered responses among those who oppose legalizing marijuana. A 2013 poll found that, among the public overall, 77% said that marijuana had “legitimate medical uses.” ↩

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9 facts about Americans and marijuana

Most americans favor legalizing marijuana for medical, recreational use, most americans now live in a legal marijuana state – and most have at least one dispensary in their county, americans overwhelmingly say marijuana should be legal for medical or recreational use, clear majorities of black americans favor marijuana legalization, easing of criminal penalties, most popular, report materials.

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The Data On Legalizing Weed

Greg Rosalsky

Editor's note: This is an excerpt of Planet Money 's newsletter. You can sign up here .

Last month, New Jersey Governor Phil Murphy signed three bills making it official: marijuana will soon be growing legally in the gardens of the Garden State for anyone over 21 to enjoy. The bills follow through on a marijuana legalization ballot initiative that New Jerseyans approved overwhelmingly last year. New Jersey is now one of a dozen states, plus the District of Columbia, which have let loose the magic dragon — and more states, like Virginia, may be on the way.

It's been almost a decade since Colorado and Washington legalized marijuana. That's given economists and other researchers enough time to study the effects of the policy. Here are some of the most interesting findings:

Legalization didn't seem to substantially affect crime rates — Proponents of legalizing weed claimed it would reduce violent crimes. Opponents said it would increase violent crimes. A study by the CATO Institute finds, "Overall, violent crime has neither soared nor plummeted in the wake of marijuana legalization."

Legalization seems to have little or no effect on traffic accidents and fatalities — Opponents of marijuana legalization argued it would wreak havoc on the road. A few studies have found that's not the case. Economists Benjamin Hansen, Keaton S. Miller & Caroline Weber, for instance, found evidence suggesting it had no effect on trends in traffic fatalities in both Colorado and Washington.

Legalization has barely affected the price of marijuana — Many people believed that marijuana prices would crash after legalization, providing an increased incentive to use it. But a recent study by the CATO institute found prices have barely budged. The price of getting high has stayed high. In California, for example, the price of marijuana actually increased after legalization, before leveling off at about $260 an ounce. Before full legalization, it cost about $250 an ounce. All the states that have legalized marijuana have seen prices converge around that level. "The convergence in prices across states is consistent with the idea that legalization diverts marijuana commerce from underground markets to legal retail shops, allowing retailers to charge a premium as the preferred sources of supply," the authors write.

Legalization has created jobs. Lots of jobs — A new report by Leafly and Whitney Economics finds the marijuana industry is booming. In 2020 alone, they calculate, it created 77,000 jobs. Across the country, there are about 321,000 jobs in the legal marijuana industry. That's more than the mining industry .

Legalization is good for state budgets — Tax revenue from legal recreational marijuana has surpassed everyone's expectations. Colorado usually collects more than $20 million a month. In 2020, the state collected a total of $387 million. The California government collects more than $50 million a month. You can find similar stories in other states that have legalized.

Legalization may be good for states' workers' comp programs — A new working paper by economists Rahi Abouk, Keshar M. Ghimire, Johanna Catherine Maclean and David Powell finds that states that legalized recreational marijuana saw a significant decline in the use of their worker compensation systems. They estimate that the number of workers aged 40-62 who received income from workers comp fell by about 20 percent following legalization. Evidence suggests that the reason for the decline is that marijuana provides "an additional form of pain management therapy" that reduces use of opioids, which are highly addictive and can be much more debilitating. Marijuana's effect on reducing opioid abuse has been documented in other studies .

Depending on whom you ask, it's not all good news. If you believe smoking marijuana is bad, then you'll be unhappy to hear that its use, naturally, increases after legalization. A study in the American Journal For Preventative Medicine , for example, found that legalization in Washington may be stalling the decades-long decline of marijuana use amongst teens. Another study found adults over 26 consume more cannabis after legalization. It seems even the AARP crowd is toking up more these days, too.

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• Washington State University

Youth cannabis vaping highest in medical marijuana states

PULLMAN, Wash. – More U.S. high school seniors reported vaping cannabis in states where it is legal only for medical purposes than states where all adult use is permitted – a study finding that surprised the researchers.

About 27% of twelfth graders in medical marijuana states reported vaping cannabis compared to 19% in states that prohibited cannabis or allowed it for adult use, according to the Washington State University study.

“More than a quarter of our youth in medical states were vaping cannabis. That’s a lot,” said Christian Maynard, a WSU sociology Ph.D. student and first author of the study published in the  journal Drug and Alcohol Dependence Reports . “We were expecting medical and adult use states would be more similar. Instead, we didn’t find any statistical difference between prohibited and adult use states.”

For this study, Maynard and his advising professor, WSU sociologist Jennifer Schwartz, analyzed responses from 3,770 high school seniors to the 2020 Monitoring the Future survey, a project which has been surveying U.S. youth since 1975.

The researchers also analyzed a subset of 556 participants who had also answered questions about access to cannabis vaping products and risk perceptions. They found that 62% of the high school seniors in medical marijuana states reported very easy access to cannabis vaping cartridges or “carts,” and only 31% saw regular cannabis use as a great risk.

In both prohibited and adult use states, fewer high school seniors, 52%, reported easy access to cartridges. More also felt regular cannabis use was risky: 40% in prohibited states and 36% in adult use states.  

The study could not identify exact reasons for the high rates of teen vaping in medical marijuana states, but Maynard suspected there may be a couple factors at play.

“It’s possible the context of saying cannabis is for medical reasons is contributing to the fact that youth view it as less risky,” said Maynard. “The difference in availability may also be that adult use states are providing legal cannabis to a wider range of people, which may in turn tamp down on the illegal market, and an adolescent can’t go to a dispensary.”

More research needs to be done to get to the reasons behind this difference, Maynard emphasized.

While cannabis and tobacco use among teens has been decreasing overall, vaping has bucked that trend. Among high school seniors, cannabis vaping during the past 30 days made the second the biggest single-year jump in 2019 for any substance in the 45-year history of the Monitoring the Future study. It was only second to nicotine vaping.

Vaping remains popular even after crisis of related lung injuries in 2019 and 2020 that led to more than 2,000 hospitalizations including 68 deaths. Many of the cases were connected to cartridges sold outside of stores that contained Vitamin E, according to the  Centers for Disease Control and Prevention .

The rise in cannabis vaping among teens highlights the need for parents and educators to help inform youth of the dangers, Maynard said.

“Like it or not, cannabis legalization seems to be happening across the country,” he said. “It’s very important to talk with adolescents. We know that at a younger age, when the brain is developing that cannabis is associated with harmful side effects. It’s also not safe to buy cannabis carts off the streets. You don’t know what they’re putting in those unregulated carts.”

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International Perspectives on the Implications of Cannabis Legalization: A Systematic Review & Thematic Analysis

Anees bahji.

1 Department of Public Health Sciences, Queen’s University, Kingston, ON K7L 3N6, Canada

2 Department of Psychiatry, Queen’s University, Kingston, ON K7L 3N6, Canada

Callum Stephenson

3 School of Kinesiology and Health Studies, Queen’s University, Kingston, ON K7L 3N6, Canada

The legality, recreational and medical use of cannabis varies widely by country and region but remains largely prohibited internationally. In October 2018, Canada legalized the recreational use of cannabis—a move many viewed as controversial. Proponents of legalization have emphasized the potential to eradicate the marijuana black market, improve quality and safety control, increase tax revenues, improve the availability of medical cannabis, and lower gang-related drug violence. Conversely, opponents of legalization have stressed concerns about cannabis’ addictive potential, second-hand cannabis exposure, potential exacerbation of underlying and established mental illnesses, as well as alterations in perception that affect safety, particularly driving. This systematic review synthesizes recent international literature on the clinical and public health implications of cannabis legalization.

1. Introduction

Globally, cannabis is the most widely used illicit drug [ 1 ], and cannabis use and dependence are estimated to have increased over the past two decades [ 2 ]. In 2016, the United Nations Office on Drugs and Crime estimated 192 million people aged 15–64 years used cannabis [ 3 ]. The Global Burden of Disease study estimated the age-standardized rate of cannabis use disorder in 2017 was 289.7 per 100,000 population (95% Uncertainty Interval (UI) 248.9–339.1), affecting 22.1 million people (95% UI 19.0–25.9 million) [ 2 ]. The United States and Canada are estimated to have among the highest age-standardized rates of cannabis use disorders (CUDs) in the world [ 2 ]. In Canada, the prevalence of cannabis use and CUDs have increased over the past decade, paralleling changes in the legal and political climate favoring legalization [ 4 , 5 ]. In 2017, 4.5 million Canadians aged 15 years or older reported past-year cannabis use, 1.5 million reported daily or near-daily use, and 6.8% met criteria for CUD [ 6 , 7 ].

People who use cannabinoids may seek varied effects that include relaxation, euphoria, relief from stress, increased appetite, improved sleep, and self-confidence [ 8 ]. However, a range of adverse physical and psychological consequences may also be experienced. The effects of short-term use include impaired short-term memory, impaired motor coordination, altered judgment, and—at higher doses—paranoia and psychoses [ 9 , 10 ]. The effects of long-term or heavy cannabinoid use include addiction, altered brain development, poor educational outcome, cognitive impairment, diminished life satisfaction and achievement, symptoms of chronic bronchitis, and increased risk of chronic psychotic disorders [ 9 ]. There are also potentially fatal harms, particularly among people who use cannabinoids regularly or who are dependent. These include risks of injuries (both unintentional and intentional, including exposure to violence), motor vehicle collisions, and suicide [ 9 , 11 ].

In recent years, recreational cannabis use has become increasingly decriminalized and legalized in many jurisdictions, including Canada in October 2018 [ 12 ]. Cannabis has also been legalized for recreational (and often medicinal use) in many U.S. states—including Colorado and California—while remaining a Schedule-I drug federally [ 13 ]. Despite this, illicit sales and use of cannabis continue to be prevalent, particularly among marginalized people who use illicit drugs [ 14 ].

Proponents of legalization have emphasized the potential to eradicate the marijuana black market [ 15 ], improve quality and safety control [ 15 ], increase tax revenues [ 16 ], improve the availability of medical cannabis [ 17 ] and lower gang-related drug violence [ 18 ]. As legalization regimes are established in multiple countries, public health professionals are increasingly synthesizing decades of knowledge from other policy areas to inform effective cannabis policy [ 13 ]. Conversely, opponents of legalization have stressed concerns about cannabis’ addictive potential [ 1 ], second-hand cannabis exposure [ 19 ], potential exacerbation of underlying and established mental illnesses [ 20 ], as well as alterations in perception and attitudes towards cannabis, particularly those that affect safety and driving.

Given these seemingly diametrically opposed views, the impact of legalization remains unclear. This is compounded by the fact that cannabis use is only legal in a handful of jurisdictions—including the Netherlands, Uruguay, Canada, and specific U.S. states. However, recent research involving cannabis has been increasing, and, studies measuring changes in cannabis-related outcomes before and after legalization are also on the rise. To date, there have been no reviews exploring the implications of cannabis legalization in jurisdictions where medical or recreational marijuana use is permitted.

The purpose of this review was to synthesize recent literature on this theme.

2.1. Reporting

We conducted a systematic review in accordance with the PRISMA guidelines [ 21 ].

2.2. Search Strategy

We searched three online databases (MEDLINE, EMBASE, and PsycINFO) for recent articles (published since 2018) exploring the clinical and public health implications of cannabis legalization.

2.3. Definition of Legalization

For the purposes of this review, a liberal definition of cannabis legalization as adopted, which we defined as legislation that permits either the medical or recreational use of cannabis in a defined geographic region [ 22 ]. A full-search strategy is provided in Appendix A.1 .

2.4. Eligibility Criteria

Articles exploring one or more implications of cannabis legalization were considered eligible, such as articles measuring the epidemiology of mental health conditions, health service utilization, rates of cannabis consumption. Articles were not excluded on the basis of study design, population considered, or geographic region, however, the review was restricted to human studies and to English language articles.

2.5. Study Selection

Citations were imported into Covidence—an online systematic review software—which facilitated the removal of duplicate citations [ 23 ]. Studies were screened by title and abstract by one author (Anees Bahji) and reviewed for consistency by a second (Callum Stephenson). Full-text articles were then reviewed by both authors for inclusion.

2.6. Data Extraction and Synthesis

Data were pooled by way of a qualitative narrative synthesis. Findings from individual studies were then grouped using thematic analysis into broad categories to facilitate meaningful discussion points. The capacity for a quantitative meta-analysis was limited due to the diverse nature of studies considered and outcomes reported.

3.1. Systematic Review

A total of 36 studies met inclusion criteria for this systematic review ( Appendix A.2 ), covering a variety of themes: prevalence and trends in cannabis consumption (n = 5); health implications (n = 9); healthcare utilization (n = 7); criminality (n = 4); cannabis black-market implications (n = 9); and health policy (n = 2).

3.2. Cannabis Consumption Implications

Although only a few studies measured the prevalence of cannabis use following legalization, most found that the prevalence increased. For example, overall marijuana consumption increased in Washington [ 24 ], while rates of marijuana use by undergraduate students increased substantially following legalization of recreational cannabis in Colorado [ 25 , 26 , 27 ]. Conversely, marijuana legalization was found to increase the prevalence of cardiovascular complications and cardiac-related deaths [ 28 ].

3.3. Health Implications

Following medical marijuana legalization in several U.S. states, there were significant increases cardiac mortality rates [ 29 ], but there were concurrent reductions in the rates of opioid prescribing, particularly in areas where cannabis dispensaries were legal. However, in these states, there was a concurrent increase in tobacco sales [ 30 ]. Compliance rates among chronic pain patients who were treated with opioids did not change following legalization in select U.S. states [ 31 ]. In areas where medical marijuana was legalized, the prevalence of serious mental illnesses—like schizophrenia and bipolar disorder—were significantly higher following legalization compared to the period before legalization [ 32 ]; however, these studies were not able to distinguish between true epidemiological rises in prevalence from increasing rates of diagnosis. While frequent cannabis use was associated with impulsivity at an individual level, there was no relationship observed between population-based cannabis use and rates of impulsive behaviour [ 33 ].

3.4. Public Opinions Towards Cannabis Legalization

In Uruguay, public views towards cannabis liberalisation were intertwined with concerns about public security and apprehension that it will open the gate to heavier drugs (like amphetamines or opioids) rather than with concerns about individual health and demographic factors [ 34 ].

3.5. Maternal/Infant/Child Health Implications

Available data suggests that cannabis use during pregnancy is relatively common and persistent, despite knowledge of the potential risks of harm [ 35 ]. However, views and perspectives toward legalization vary among pregnant women and may impact cannabis use during pregnancy [ 36 ]. Similarly to the general population, rates of marijuana use have increased after marijuana legalization among pregnant and parenting women in Washington [ 37 ]. However, there were no changes in the prevalence of low birth weight or small for gestational age births during this same interval [ 38 ]. Despite these findings, the risks of prenatal cannabis exposure should be neither overstated nor minimized, and that the legal status of a substance should not be equated with safety [ 39 ]. Scientifically accurate dissemination of cannabis outcomes data is necessary [ 40 ], and clinicians must recognize that even in environments where cannabis is legal, pregnant women may end up involved with children’s protective services [ 41 ].

3.6. Healthcare Utilization Implications

Retrospective studies have observed notable increases in the number and rates of emergency department (ED) visits for cannabis-related presentations after recreational marijuana legalization in the United States [ 42 ]. Likewise, the prevalence of psychiatric comorbidity is substantially higher in adults presenting to the ED for cannabis-associated diagnostic codes than for other visits [ 43 ]. Rates of hospitalization (32.9% vs. 18.9%) and the ED length stay (3 vs. 2 hours) tend to be higher for visits involving inhaled cannabis than edible cannabis [ 44 ]. These differences might indicate greater severity of adverse events with inhalable cannabis, but they also may also reflect differences in the underlying clinical presentation [ 44 ].

3.7. Criminal Implications

In a retrospective analysis comparing crime data in neighbouring American states, Washington and Oregon, revealed that reported rape, property crimes, and thefts all decreased by 15–30%, 10–20%, and 13–22% respectively after legalization in Washington [ 24 ]. Moreover, the prevalence of these crimes occurring across the border where recreational cannabis use is illegal, in Oregon, remained the same. Furthermore, in 2018, all divisions of police clearance rates either remained stagnant or improved in Colorado and Washington following recreational marijuana legalization [ 45 ]. Marijuana-related arrests in Washington decreased from 5,531 in 2012 to 120 in 2013, allowing for more police resource allocation to other divisions [ 45 ]. However, there was no alteration in the racial disparity of marijuana related arrests after legalization, with African Americans still accounting for 2.7 times more arrests than Caucasians [ 46 ].

3.8. Cannabis Black Market Implications

Although legalization was intended to reduce illicit cannabis sales, the black market for cannabis in Canada has actually increased with legalization—not decreased [ 47 ]. This has occurred largely because more marijuana is available from legal sources to sell illegally [ 48 ]. According to preliminary reports from Statistics Canada, 79% of cannabis was bought illegally in the fourth quarter of 2018, down from 90% in the third quarter [ 49 ]. Recently released reports indicate that Canadians buying cannabis from legal sources pay about $10 per gram, while those who have stuck with the grey market pay $6.37 per gram [ 49 ].

U.S. states without legalized marijuana bordering states with legalized marijuana are thought to have increased availability of marijuana for the black market [ 50 , 51 ]. For example, despite legalizing cannabis in 2016, California still has a thriving black market, with as much as 80% of all cannabis sales being linked to illegal sources [ 52 , 53 ]. Recent economic estimates suggest that California’s illicit cannabis market is worth approximately $3.7 billion—more than four times the size of the legal market in the state [ 54 , 55 ].

3.9. Policy Implications

One study reported a risk–benefit framework on the impact of legalization on mental disorders—drawing on the impact of cannabis use on incidence, prevalence and severity of mental disorders [ 56 ]. Similarly, the Canadian Psychiatric Association released a position statement outlining potential concerns on the implications of legalization on mental health [ 40 ], highlighting concerns about early exposure and abnormal brain development.

4. Discussion

4.1. summary of findings.

This review identified 36 studies exploring diverse aspects of cannabis legalization—health, epidemiology, health service utilization, public policy, crime, and economic implications. Although only a few studies measured the prevalence of cannabis use following legalization, most found that the prevalence of cannabis use increased over time. Available data also suggests that cannabis use during pregnancy is relatively common and persistent, despite knowledge of the potential risks of harm—however, the long-term effects of cannabis exposure in utero remains unclear. Legalization of marijuana in the regions of the United States was followed by increases in the number and rates of emergency department visits for cannabis-related presentations (such as cannabis intoxication and cannabis-related hyperemesis). From an economic perspective, early studies indicate that legalization—which was intended to reduce illicit cannabis sales—may have created a nexus for a stronger illicit black market for cannabis sales in Canada, which have actually increased post-legalization. Still, cannabis-related criminal activity has reduced in certain regions where cannabis has been legalized.

4.2. Significance of Findings

Collectively, the effects of cannabis legalization are incredibly heterogeneous, however, research is beginning to demystify certain beliefs about cannabis in the era of legalization. As such, research involving the implications of cannabis legalization is also increasing at a rapid rate. However, there is still significant controversy regarding the overall impact of legalization—particularly on mental health and public policy. As such, cannabis policy is rapidly evolving in Canada, the United States, and the rest of the world as more jurisdictions legalize medical and recreational marijuana use.

Overall, public opinion has shifted dramatically in favor of marijuana legalization, particularly in the United States. Cannabis use is also on the rise even among older adults, who have historically been left out of discussions pertaining the cannabis and drug policy in general. Strikingly, the increasing prevalence of cannabis use occurs in the background of social perceptions that cannabis is associated with a relatively low associated risk, which diametrically opposes current knowledge about the biological and clinical effects of cannabis use in both the short and long term [ 57 ].

4.3. Limitations

Although this review has notable strengths, the findings presented here should be interpreted in light of some significant limitations. First, as a systematic review, the quality and availability of the evidence presented is limited by the extent of published literature—this is particularly relevant given that extent of cannabis legalization worldwide is fairly limited, which precludes extensive research on this topic. Second, as this review focused primarily on recent literature (published since 2017), several key articles that were published prior to the search date were excluded from the discussion. As such, the contributions of research articles not discussed in this review should not be discounted because this review did not mention them. For example, there is a much larger well of knowledge on cannabis legalization and criminal activity than the selection of articles covered by this review. Third, given the scope of this topic, a formal quantitative meta-analysis was not conducted—this may be undertaken in future studies focusing on specific aspects of cannabis legalization (such as the prevalence of specific emergency room visits). Fourth, our review excluded non-English language studies, which may have precluded the inclusion of key articles from non-English speaking jurisdictions, such as Uruguay.

5. Conclusions

Despite changing legal climates, there is an increasing demand for clear and consistent messaging on the effects of cannabis use. Currently, there is a paucity of literature on a variety of implications related to cannabis legalization—and the available studies are fairly heterogeneous in their findings. As such, clear conclusions are difficult to draw at this point in time. However, with legalization and liberalization of cannabis underway in many parts of the world, it is likely that the answers to these questions will become available in the near future. Thus, the ongoing accumulation of empirical data will be helpful to inform ongoing debates about the role of public policy on cannabis legislation.

Appendix A.1. Search Strategy

MEDLINE: 2018—present

• exp Legislation, Drug/31216
• cannabis.mp. or exp Cannabis/20238
• exp Marijuana Abuse/or exp Marijuana Smoking/or exp Medical Marijuana/10236
• 2 or 3/24815
• 1 and 4/1306
• Limit 5 to (english language and humans and yr = ”2018 − Current”)/72

EMBASE: 2018—present

• exp drug legalisation/15125
• exp “cannabis use”/or exp cannabis/or exp cannabis smoking/or exp medical cannabis/40,431
• 1 and 2/871
• Limit 5 to (english language and humans and yr = “2018 – Current”)/54

PsycINFO: 2018—present

• exp drug legalization/668
• exp Cannabis or cannabis.mp/13,302
• 1 and 2/366
• Limit 5 to (english language and humans and yr = “2018 – Current”)/25

Appendix A.2. Study Flow Diagram

Author Contributions

Conceptualization, A.B. and C.S.; methodology, A.B. and C.S.; software/validation/formal analysis, A.B. and C.S.; investigation, A.B. and C.S.; resources, A.B. and C.S.; data curation, A.B. and C.S.; writing—original draft preparation, A.B. and C.S.; writing—review and editing, A.B. and C.S.; visualization, A.B. and C.S.; supervision, A.B.; project administration, A.B.; funding acquisition, not applicable.

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

Deserted: The U.S. Military's Sexual Assault Crisis as a Cost of War

Over the past decade, the U.S. military has implemented policies to promote gender equality, notably lifting the ban on women in combat roles in 2013 and opening all military jobs to women by 2016. Yet, even as U.S. military policy reforms during the “War on Terror” appear to reflect greater equality, violent patterns of abuse and misogyny continued within military workplaces.

This author of this report found that sexual assault prevalence in the military is likely two to four times higher than official government estimations. Based on a comparison of available data collected by the U.S. Department of Defense to independent data, the research estimates there were 75,569 cases of sexual assault in 2021 and 73,695 cases in 2023. On average, over the course of the war in Afghanistan, 24 percent of active-duty women and 1.9 percent of active-duty men experienced sexual assault. The report highlights how experiences of gender inequality are most pronounced for women of color, who experience intersecting forms of racism and sexism and are one of the fastest-growing populations within the military. Independent data also confirm queer and trans service members’ disproportionately greater risk for sexual assault.

The report notes that during the post-9/11 wars, the prioritization of force readiness above all else allowed the problem of sexual assault to fester, papering over internal violence and gender inequalities within military institutions.

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