Review of: Jeremy Mennis et al., “Treatment admissions for opioids, cocaine, and methamphetamines among adolescents and emerging adults after legalization of recreational marijuana,” Journal of Substance Abuse Treatment 122 (2021), p. 108228.

Reviewed by: Stacey McKenna, PhD

As of March 2021, 15 states and Washington, D.C. have voted or passed legislation to legalize and regulate adult use of marijuana. Opponents of such policies often cite concerns that cannabis is a “gateway” drug, and claim that its legalization will lead not only to increases in marijuana consumption, but also in the use and abuse of other drugs. Indeed, the vast majority of illicit drug users have used or currently use cannabis; and marijuana use does often precede use of other drugs. However, according to the Centers for Disease Control and Prevention, this concept of cannabis as a “gateway” drug is not sufficiently supported by research. In fact, most people who use marijuana will never move on to other illicit substances. Furthermore, because much of the research has been correlational, no conclusions can be drawn about causation. Explanations for the association range from chemical changes in the brain resulting from early cannabis use to a general predisposition for drug use. In brief, much about the relationship remains unknown or misunderstood.

Thus, as legalization of recreational cannabis spreads across the country, researchers are taking advantage of what may be considered a “natural laboratory” to examine the impacts of such policies. In a 2021 paper published in the Journal of Substance Abuse and Treatment, Jeremy Mennis and colleagues investigated whether the legalization of recreational marijuana was associated with subsequent increases in substance abuse disorder (SUD) treatment admissions for cocaine, opioids and methamphetamines among youth.

To examine this question, the researchers drew on data from the U.S. Substance Abuse and Mental Health Services Administration’s (SAMHSA) Treatment Episode Dataset-Admissions (TEDS-A). They compared Colorado and Washington’s trends in SUD treatment admissions before (2008-2012) and after (2013-2016) legalization with the same time period in states that had not legalized recreational cannabis. They excluded nine states that implemented recreational marijuana legalization during the study period or that lacked data on specific substances.

Mennis et al. used “391 annual observations of cocaine, opioid, and methamphetamine treatment admissions rates for 41 states” to build a series of models. They ran separate models for each drug and age group (adolescents 12-17 years, emerging adults 18-20 years and late emerging adults 21-24 years), resulting in nine distinct analyses. Rather than running individual state comparisons, their primary models compared overall trends in recreational marijuana legalization (RML) states versus non-RML states.

The unadjusted models did indicate that Colorado and Washington had relatively higher adolescent admission rates across all three substances compared to the non-RML states. However, admission trajectories were not significantly different, either prior to legalization (2008-2012), or in a pre- versus post-legalization comparison.

Interestingly, when the researchers separated the two RML states—Colorado and Washington—they found that their trajectories were very different from one another. When Colorado was independently compared to the non-RML states, they did observe relative increases in methamphetamine and opioid treatment admissions among adolescents following the legalization of recreational marijuana. In Washington, however, adolescent treatment admissions decreased significantly after the policy was enacted.

Based on these findings, the authors conclude that the “legalization of recreational marijuana use in Colorado and Washington did not result in an increase in SUD treatment admissions for cocaine, opioids, or methamphetamines among adolescents or emerging adults.” They offer the following potential explanations for their findings:

However, there are a number of limitations and areas in need of further investigation.

Mennis et al. recognize that there is likely a need for additional studies that take into account the complex differences between regions and jurisdictions as a number of factors beyond policy—such as cultures of acceptance, regional influence and drug availability—are likely shaping individuals’ drug use behaviors and statewide trajectories. For example, they note that a culture of acceptance likely played a role in RML in Washington and Colorado. As such, youth may have already had increased exposure and access prior to the policy shift. Other weaknesses of the current study include its exclusive focus on publicly funded treatment centers, the failure to account for discrete data collection practices and the relatively short period of time between the enactment of RML in Colorado and Washington and the collection of this data.

Nonetheless, this study is among the first to directly assess the potential impact of RML on SUD treatment admissions. As such, it makes an important contribution to early understandings of how such policies affect health in the real world.

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