Do Medical Marijuana Laws Increase Marijuana Use? Replication Study and Extension

doi:10.1016/j.annepidem.2011.12.002

Annals of Epidemiology

Volume 22, Issue 3, March 2012, Pages 207-212

https://doi.org/10.1016/j.annepidem.2011.12.002 Get rights and content

Purpose

To replicate a prior study that found greater adolescent marijuana use in states that have passed medical marijuana laws (MMLs), and extend this analysis by accounting for confounding by unmeasured state characteristics and measurement error.

Methods

We obtained state-level estimates of marijuana use from the 2002 through 2009 National Survey on Drug Use and Health. We used 2-sample t-tests and random-effects regression to replicate previous results. We used difference-in-differences regression models to estimate the causal effect of MMLs on marijuana use, and simulations to account for measurement error.

Results

We replicated previously published results showing higher marijuana use in states with MMLs. Difference-in-differences estimates suggested that passing MMLs decreased past-month use among adolescents by 0.53 percentage points (95% confidence interval [CI], 0.03–1.02) and had no discernible effect on the perceived riskiness of monthly use. Models incorporating measurement error in the state estimates of marijuana use yielded little evidence that passing MMLs affects marijuana use.

Conclusions

Accounting for confounding by unmeasured state characteristics and measurement error had an important effect on estimates of the impact of MMLs on marijuana use. We find limited evidence of causal effects of MMLs on measures of reported marijuana use.

Introduction

The potential impact of legalizing medical marijuana on both medical and recreational marijuana use has received much popular and legislative attention (1), but little empirical study. In a recent issue of the Annals, Wall et al. contributed to this literature by analyzing the prevalence of marijuana use among adolescents in US states that have and have not passed a law legalizing marijuana for medical purposes (2). They reported evidence that rates of marijuana use were higher in states that had passed medical marijuana laws (MMLs) compared with states that had not passed laws, but concluded that the causal mechanism could not be determined. In this paper, we replicate the analyses of Wall et al. and, using the same data, we estimate the causal effect of passing MMLs on measures of marijuana use.

Section snippets

Methods

Wall et al. were transparent with respect to both their data and methods, which greatly facilitated replicating their results. We abstracted data from 2002 through 2009 on the state-level prevalence of past-month marijuana use and perceived riskiness of monthly marijuana use from the publicly available estimates of the National Survey on Drug Use and Health (NSDUH) provided by the US Substance Abuse and Mental Health Survey Administration (3). Additional details on the survey methodology are

Results

Table 1 shows comparisons between the estimates presented by Wall et al. (2) and our own estimates using the same data. We find nearly identical prevalence estimates of past-month marijuana use and the perceived riskiness of monthly marijuana use. Our replication of their 2-sample t-tests provides similar evidence that states with MMLs have higher average rates of marijuana use and lower perceived riskiness of monthly use than states without laws in each data year. As Wall et al. reported, from

Discussion

The evidence presented by Wall et al. suggested that states passing MMLs since 2004 had higher prevalence of marijuana use than states not passing laws. They gave 3 possible explanations for this finding: (i) States with higher use are more likely to enact laws, (ii) laws increase use, and (iii) unmeasured factors (e.g., social norms about drug use) that affect marijuana use and affect the likelihood of passing a law may be more common in states that passed laws. Our analysis, using the same

Conclusion

We replicated the findings of Wall et al. (2) that marijuana use was higher in states that have passed MMLs, and our analysis suggests this is unlikely to be a causal association. Our difference-in-differences estimates suggest little detectable effects of passing MMLs on marijuana use or perceived riskiness of use among adolescents or adults, which is consistent with some limited prior evidence on arrestees and emergency department patients (17). Future analyses that take advantage of

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Do Medical Marijuana Laws Increase Marijuana Use? Replication Study and Extension

Purpose

Methods

Results

Conclusions

Introduction

Section snippets

Methods

Results

Discussion

Conclusion

Ann Epidemiol

Int J Drug Policy

Multilevel and longitudinal modeling using Stata

Natural and quasi-experiments in economics

Journal of Business & Economic Statistics

Mostly harmless econometrics: an empiricist’s companion

The case-crossover design: A method for studying transient effects on the risk of acute events

Am J Epidemiol