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Meta-Analysis
. 2018 Jun;113(6):1003-1016.
doi: 10.1111/add.14136. Epub 2018 Feb 22.

Medical Marijuana Laws and Adolescent Marijuana Use in the United States: A Systematic Review and Meta-Analysis

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Free PMC article
Meta-Analysis

Medical Marijuana Laws and Adolescent Marijuana Use in the United States: A Systematic Review and Meta-Analysis

Aaron L Sarvet et al. Addiction. .
Free PMC article

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.

Keywords: Adolescent; epidemiology; marijuana; meta-analysis; policy; systematic review.

Figures

Figure 1
Figure 1
Systematic search flow‐chart
Figure 2
Figure 2
Years of coverage, and overlap with passage of state medical marijuana laws (MMLs), of studies included in meta‐analysis. MTF = Monitoring the Future Study; NLSY = National Longitudinal Survey of Youth; NSDUH = National Survey on Drug Use and Health; YRBS = Youth Risk Behavior Survey. [Colour figure can be viewed at http://wileyonlinelibrary.com]
Figure 3
Figure 3
Fixed‐effects meta‐analysis: pre–post medical marijuana law (MML) change in past‐month marijuana prevalence within MML states. MTF = Monitoring the Future Study; NLSY = National Longitudinal Survey of Youth; NSDUH=National Survey on Drug Use and Health; YRBS = Youth Risk Behavior Survey; FE = fixed‐effects. FE model: θ¯=i=111wiθi/i=111wi, where θ¯ corresponds to the final fixed‐effects estimate, θi corresponds to the i th study's standardized estimate and wi corresponds to the inverse of the variance of the i th study's standardized estimate

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