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Aims: Medical marijuana law (MML) enactment in the United States has been associated with increased cannabis use but lower traffic fatality rates. We assessed the possible association of MML and individual‐level driving under the influence of cannabis (DUIC) and also under the influence of alcohol (DUIA). Design and setting: Three cross‐sectional U.S. adult surveys: The National Longitudinal Alcohol Epidemiologic Survey (NLAES; 1991–1992), the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC; 2001–2002), and the NESARC‐III (2012–2013). Participants: The total n was 118 497: 41 764, 41 184, and 35 549 from NLAES, NESARC, and NESARC‐III, respectively. Measurements Across the three surveys, similar questions in the Alcohol Use Disorder and Associated Disabilities Interview Schedule assessed DUIC and DUIA. Ever‐MML states enacted MML between 1991–1992 and 2012–2013 (overall period). Early‐MML states enacted MML between 1991–1992 and 2001–2002 (early period). Late‐MML states enacted MML between 2001–2002 and 2012–2013 (late period). MML effects on change in DUIC and DUIA prevalence were estimated using a difference‐in‐differences specification to compare changes in MML and other states. Findings From 1991–1992 to 2012–2013, DUIC prevalence nearly doubled (from 1.02% to 1.92%), increasing more in states that enacted MML than other states (difference‐in‐differences [DiD] = 0.59%; 95% CI = 0.06%–1.12%). Most change in DUIC prevalence occurred between 2001–2002 and 2012–2013. DUIC prevalence increased more in states that enacted MML 2001–2002 to 2012–2013 than in never‐MML states (DiD = 0.77%; 95% CI = −0.05%‐1.59%), and in two early‐MML states, California (DiD = 0.82; 95% CI = 0.06–1.59) and Colorado (DiD = 1.32; 95% CI = 0.11–2.53). In contrast, DUIA prevalence appeared unrelated to MML enactment. Conclusions: Medical marijuana law enactment in US states appears to have been associated with increased prevalence of driving under the influence of cannabis, but not alcohol. [ABSTRACT FROM AUTHOR]

Abstract: Aims: To conduct a systematic review and meta‐analysis of studies in order to estimate the effect of US medical marijuana laws (MMLs) on past‐month marijuana use prevalence among adolescents. Methods: A total of 2999 papers from 17 literature sources were screened systematically. Eleven studies, developed from four ongoing large national surveys, were meta‐analyzed. Estimates of MML effects on any past‐month marijuana use prevalence from included studies were obtained from comparisons of pre–post MML changes in MML states to changes in non‐MML states over comparable time‐periods. These estimates were standardized and entered into a meta‐analysis model with fixed‐effects for each study. Heterogeneity among the study estimates by national data survey was tested with an omnibus F‐test. Estimates of effects on additional marijuana outcomes, of MML provisions (e.g. dispensaries) and among demographic subgroups were abstracted and summarized. Key methodological and modeling characteristics were also described. Preferred Reporting Items for Systematic Reviews and Meta‐Analyses (PRISMA) guidelines were followed. Results: None of the 11 studies found significant estimates of pre–post MML changes compared with contemporaneous changes in non‐MML states for marijuana use prevalence among adolescents. The meta‐analysis yielded a non‐significant pooled estimate (standardized mean difference) of −0.003 (95% confidence interval = −0.012, +0.007). Four studies compared MML with non‐MML states on pre‐MML differences and all found higher rates of past‐month marijuana use in MML states pre‐MML passage. Additional tests of specific MML provisions, of MML effects on additional marijuana outcomes and among subgroups generally yielded non‐significant results, although limited heterogeneity may warrant further study. Conclusions: Synthesis of the current evidence does not support the hypothesis that US medical marijuana laws (MMLs) until 2014 have led to increases in adolescent marijuana use prevalence. Limited heterogeneity exists among estimates of effects of MMLs on other patterns of marijuana use, of effects within particular population subgroups and of effects of specific MML provisions. [ABSTRACT FROM AUTHOR]

Aims To test, among US students: (1) whether perceived harmfulness of marijuana has changed over time, (2) whether perceived harmfulness of marijuana changed post-passage of state medical marijuana laws (MML) compared with pre-passage; and (3) whether perceived harmfulness of marijuana statistically mediates and/or modifies the relation between MML and marijuana use as a function of grade level. Design Cross-sectional nationally representative surveys of US students, conducted annually, 1991-2014, in the Monitoring the Future study. Setting Surveys conducted in schools in all coterminous states; 21 states passed MML between 1996 and 2014. Participants The sample included 1 134 734 adolescents in 8th, 10th and 12th grades. Measurements State passage of MML; perceived harmfulness of marijuana use (perceiving great or moderate risk to health from smoking marijuana occasionally versus slight or no risk); and marijuana use (prior 30 days). Data were analyzed using time-varying multi-level regression modeling. Findings The perceived harmfulness of marijuana has decreased significantly since 1991 (from an estimated 84.0% in 1991 to 53.8% in 2014, P < 0.01) and, across time, perceived harmfulness was lower in states that passed MML [odds ratio (OR) = 0.86, 95% confidence interval (CI) = 0.75-0.97]. In states with MML, perceived harmfulness of marijuana increased among 8th graders after MML passage (OR = 1.21, 95% CI = 1.08-1.36), while marijuana use decreased (OR = 0.81, 95% CI = 0.72-0.92). Results were null for other grades, and for all grades combined. Increases in perceived harmfulness among 8th graders after MML passage was associated with ~33% of the decrease in use. When adolescents were stratified by perceived harmfulness, use in 8th graders decreased to a greater extent among those who perceived marijuana as harmful. Conclusions While perceived harmfulness of marijuana use appears to be decreasing nationally among adolescents in the United States, the passage of medical marijuana laws (MML) is associated with increases in perceived harmfulness among young adolescents and marijuana use has decreased among those who perceive marijuana to be harmful after passage of MML. [ABSTRACT FROM AUTHOR]