Young adults' simultaneous use of alcohol and marijuana linked to more negative consequences

Up to one in four young adults use alcohol and marijuana simultaneously (i.e., use at the same time with overlapping effects), a behavior linked to a greater risk of adverse consequences. Given the expanding legalization of non-medical marijuana use, there is an urgent need to better understand the effects of simultaneous use and who is most vulnerable to adverse outcomes. Previous research has yielded mixed and limited results.

For the study in Alcohol: Clinical and Experimental Research, investigators sought to clarify the associations and consequences of simultaneous use in young adults.

Young adults' simultaneous use of alcohol and marijuana on a given day was associated with consuming more drinks, experiencing more negative consequences from alcohol, and experiencing specific negative marijuana consequences, according to a new study. These findings relate specifically to the simultaneous use of alcohol and marijuana rather than simply the amounts consumed.

Researchers collected data from 409 adults aged 18–25 (50% women, 48% White) in the greater Seattle area who reported recent simultaneous alcohol and marijuana use. Notably, the sample included both college and non-college students. In six bursts over two years, young adults completed 14 days of online surveys, reporting their alcohol and marijuana use and related effects for the previous day. The researchers explored associations between simultaneous alcohol and marijuana use and negative consequences.

Participants reported drinking on 36% of survey days with a mean of 3½ drinks; on 28% of those days, they experienced at least one negative alcohol consequence. They reported using marijuana on 36% of survey days with a mean of three hours high and at least one negative marijuana consequence on 56% of those days.

Simultaneous alcohol and marijuana use was reported on 15% of survey days. On days of simultaneous use, participants consumed 37% more drinks and 43% more negative alcohol consequences (e.g., doing something embarrassing, feeling clumsy or confused) compared to drinking days without simultaneous marijuana use.

Further, on days of simultaneous use, participants reported being high for 10% more hours (but did not experience more negative consequences) compared to marijuana use days without simultaneous alcohol use. On days with marijuana use, each additional hour high was linked to 14% more negative marijuana consequences, and simultaneous use carried greater odds of feeling clumsy or dizzy.

Researchers note the findings may not generalize to all populations, given the sample was higher-risk substance-using young adults. The findings provide the strongest evidence to date that simultaneous use is high risk and should be the focus of ongoing education and prevention efforts—including messaging that those who use alcohol and marijuana simultaneously would particularly benefit from monitoring and limiting the amounts consumed.