Full length articleMedical marijuana laws and adolescent use of marijuana and other substances: Alcohol, cigarettes, prescription drugs, and other illicit drugs
Introduction
Since 1996, 29 states and Washington, D.C. (as of November 2017) enacted legislation permitting the medical use of marijuana. While multiple studies examined the impact of medical marijuana legalization (MML) on adolescent marijuana use (Harper et al., 2012, Hasin et al., 2015, Wall et al., 2011), less is known about MML effects on adolescent use of other substances. If marijuana and other substances are complementary, then increased marijuana use (e.g., through changes in availability and/or price) should increase alcohol and other substance use. This could occur if combined use produces a synergistic psychoactive effect or if marijuana serves as a gateway to other drugs (Kandel and Kandel, 2015, Kandel et al., 1992, Moore, 2010). Alternatively, if marijuana and other substances are substitutes, then increased marijuana use should decrease other substance use, which could occur if the substances have similar psychoactive properties and marijuana becomes more accessible. These relationships are usually investigated when a policy directly affects the substance it targets. Virtually all evidence indicates no effect of state MML on adolescent marijuana use (Anderson et al., 2013, Choo et al., 2014, Harper et al., 2012, Hasin et al., 2015, Lynne-Landsman et al., 2013, Schuermeyer et al., 2014, Wall et al., 2011, Wen et al., 2015). However, Colorado studies show that a 2009 federal policy change (reduction in likelihood of prosecution for medical use in MML states; Ogden, 2009) was associated with increased use among adolescents (Salomonsen-Sautel et al., 2014, Schuermeyer et al., 2014). This suggests possible heterogeneous MML effects across periods, warranting exploration of cross-substance effects. Further, prior to enactment, MML states have higher rates of marijuana use than non-MML states (Hasin et al., 2015, Keyes et al., 2016, Martins et al., 2016, Wall et al., 2016, Wall et al., 2011), which, according to the gateway hypothesis, could contribute to later initiation of other substances.
Studies of abused substances as substitutes or complements mainly focus on alcohol and marijuana. Some found substitution after change in alcohol prices (Cameron and Williams, 2001, Chaloupka and Laixuthai, 1997) and in the legal drinking age (Crost and Guerrero, 2012, DiNardo and Lemieux, 2001), while others showed complementarity after change in alcohol price (Pacula, 1998, Saffer and Chaloupka, 1999, Williams et al., 2004) and legal drinking age (Yoruk and Yoruk, 2011b). The two studies that examined MML and adolescent use of alcohol and other non-marijuana substances also did not agree. One study (Pacula et al., 2013) did not show MML effects overall on alcohol use, but found a positive relationship between home cultivation provisions and alcohol use and between dispensary provisions and alcohol treatment admissions. This study’s inconsistent results were only found at the extremes of alcohol severity. Further, each dataset included different states and age groups, making comparisons difficult. In the second study (Wen et al., 2015), MML did not affect alcohol or other substance use (cocaine, heroin, non-medical use of prescription opioids) among those aged 12–20. However, this study included only eight years and a small number of MML states. Thus, the evidence is inconsistent, warranting further study.
Some studies show a relationship between MML enactment and adult decreases in opioid-related harm (Bachhuber et al., 2014, Kim et al., 2016, Pacula et al., 2015), which is potentially explained if marijuana provides a substitute for opioids to treat chronic pain, ameliorate opioid withdrawal symptoms, or assist in recovery from opioid dependence (Lynch and Ware, 2015, Scavone et al., 2013). However, no prior studies examined MML effects on non-medical prescription opioid use across the full range of MML states or addressed MML effects on other types of prescription drugs.
We know of no study that examined pre-post MML differences in adolescent binge drinking and use of other non-marijuana substances in all 48 contiguous U.S. states with data that pre-dated the first MML. Our study addressed two questions. First, were participants generally at higher risk for use of marijuana, cigarettes, non-medical use of prescription drugs, illicit drugs, or binge drinking in states that ever passed a MML by 2015 than in other states? This question extends our prior work by examining whether states that ever passed MML are generally at higher risk for use of a wide spectrum of substances. Second, did states that enacted MML exhibit greater change in the prevalence of marijuana use, cigarette use, non-medical prescription drug use, illicit drug use or binge drinking following MML enactment than states that never enacted MML? While results for marijuana were previously reported (Hasin et al., 2015), we include them here because of the additional year of data (2015) available and to evaluate whether MML impacts marijuana and other substances the same way (complementarity) or in opposite ways (substitution). As a sensitivity analysis, we also examined potential differences in the effect of MML enactment on substance use following the 2009 change in federal prosecution policy in MML states (Ogden, 2009).
Section snippets
Study design and participants
Monitoring the Future (MTF) is an annual, nationally representative, cross-sectional survey of students attending public and private schools in the 48 contiguous U.S. states (Bachman et al., 2015, Miech et al., 2016). Consistent design methodology from its inception allows robust examination of historical trends. Since 1991, students were sampled from 8th, 10th and 12th grades.
MTF employs a complex survey design; from a sample of randomly selected geographic units, eligible schools are selected
Results
First, we examined whether adolescents were generally at higher risk for substance use in states that ever enacted MML by 2015. Among 10th and 12th graders, marijuana use in the prior 30 days was more prevalent before MML was enacted in states that passed MML (Table 1: difference in proportions, MML vs. non-MML states: 10th graders: 2.9%; 12th graders: 4.8%; p-values: 0.0017–0.0278). In contrast, among 10th graders, cigarette use, non-medical prescription amphetamine use and non-medical
Discussion
We presented national evidence on the impact that enactment of medical marijuana laws had on marijuana use, binge drinking, cigarette use, non-medical prescription drug use, and illicit drug use among 8th, 10th, and 12th graders. The impact of MML enactment on substance use differed by grade. Among 8th graders, marijuana use, binge drinking, cigarette use, non-medical prescription opioid, amphetamine, and tranquilizer use, and other illicit drug use decreased following MML enactment. Among 10th
Contributors
Cerdá designed the study, interpreted results, and wrote the manuscript; Wall designed the analytic plan; Sarvet, Feng and Wall conducted analyses and wrote the methods and results sections of the manuscript; Keyes and Galea participated in study design and provided input into interpretation of the study results and manuscript drafts; and Hasin provided critical input into study design, design of the analytic plan, interpretation of results, and reviewed manuscript drafts. All authors approved
Role of funding source
Nothing declared.
Conflict of interest
The authors have no conflicts of interest to declare.
Acknowledgements
This work was supported by the National Institutes of Health (R01DA034244, R01DA040924, K01DA030449, K01AA021511, T32DA031099) and by the New York State Psychiatric Institute. Funders had no role in study design, data collection, analysis, writing of the manuscript, or decision to submit to this journal. We thank Dr. Rosalie Pacula, Dr. Patrick O’Malley, and Dr. John Schulenberg for their invaluable feedback on this manuscript. This study was funded by R01DA034244 (Hasin), R01DA040924 (Cerdá),
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