Young Adults' Concurrent and Simultaneous Use of Marijuana and Alcohol in Diverse Legal Contexts

Introduction: The legal context for marijuana use in the U.S. is becoming more permissive. Past year marijuana use among young adults is rising while alcohol use remains high, raising concerns about increasing concurrent use (use of both substances in the past month, CAM) and simultaneous use (use of both substances at the same time so the effects overlap, SAM). Both forms of polysubstance use increase risks for impaired cognitive and motor functioning and high risk behavior such as driving while intoxicated (DWI). Laws allowing for medical marijuana use have been associated with increases in adult marijuana use, binge drinking, and SAM, but little is known about the possible associations between the more recent legalization of non-medical marijuana use and CAM or SAM. We examined prevalence of CAM and SAM in a longitudinal panel of contemporary young adults (age 23 in 2016) living in various legal marijuana contexts and the relation to DWI with alcohol, marijuana, or both.

Methods: We used data from a gender-balanced cohort of young adults (n = 1728) who grew up in 12 control communities of the Community Youth Development Study. Specific and overlapping use were based on participants' report of their past-month alcohol, marijuana, and simultaneous use as well as their reports of driving under the influence of either drug in the past month. Marijuana legal context was based on the state where they lived at the time of the age 23 survey in 2016 (i.e., legalized medical and nonmedical use [34.5%, n=586], legalized medical use only [7.1%, n=121], and not legalized [58.4%, n=993]).

Results: About a third of the sample reported using neither substance, 39 % used only alcohol, 4.6% used only marijuana, 7.5% reported CAM, and 12.4% reported SAM. Of all marijuana users, 82% were CAM or SAM users, whereas only 34% of all alcohol users were CAM or SAM users. SAM use was more likely among those who lived where medical marijuana (12.5%) and where non-medical marijuana (17.4%) were legal compared to where both forms were illegal (9.5%; X2= 22.4, p<.000). SAM users were twice as likely as concurrent users to DWI with alcohol (35% vs 17%; X2=157, p<.000) and marijuana (58% vs 23%; X2=511, p<.000). Of SAM users 21% reported DWI under the simultaneous influence of both substances. Follow-up analyses will control for relevant socio-demographic covariates and clustering of individuals in their original communities.

Conclusion: Young adults are more likely to use alcohol and marijuana simultaneously where medical or nonmedical marijuana is legal. Given that SAM users have an increased risk of DWI compared to concurrent users, prevention efforts targeting harm reduction in "legal" states should include avoiding simultaneous use.