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.