College freshmen who used both marijuana and alcohol had more blackouts, injuries, and DUIs than drinkers who didn't use marijuana

What the researchers found

Researchers used latent profile analysis to identify four distinct groups among 772 incoming college freshmen based on their alcohol and marijuana use patterns. The groups were: light drinking/no marijuana (40%), moderate drinking/no marijuana (25%), moderate drinking/marijuana (22%), and heavy drinking/marijuana (14%).

The most informative comparison was between the two moderate drinking groups. Those who also used marijuana were more likely to drink more than intended, drink to get drunk, and experienced more problems including higher rates of blackouts, physical injuries, and DUI, compared to moderate drinkers who did not use marijuana.

These differences occurred despite similar alcohol consumption levels, suggesting that marijuana use adds independent risk beyond what alcohol alone produces.

Why it matters

College prevention programs often focus on alcohol alone, but this study shows that combined marijuana-alcohol use creates additional risk. Students who use both substances experience more severe consequences even at the same drinking level, highlighting the need for polysubstance-focused interventions.

The numbers in context

772 freshmen. 4 groups: light/no MJ (40%), moderate/no MJ (25%), moderate/MJ (22%), heavy/MJ (14%). Moderate drinkers+MJ had more: blackouts, injuries, DUI, drinking to get drunk vs. moderate drinkers without MJ.

How the study worked

Latent profile analysis of 772 entering college freshmen (53% male, 60% White, mean age 18). Four groups identified based on alcohol and marijuana use patterns. Pairwise contrasts examined cross-class differences in demographics, drinking behaviors, and consequences.

What this study cannot tell us

Cross-sectional data from a single university at one time point. Self-reported substance use and consequences. The study cannot determine whether marijuana directly causes additional problems or whether the same risk-taking tendencies drive both marijuana use and worse outcomes.

How to read the evidence

Cross-sectional latent profile analysis with a reasonably large freshman sample. Cannot establish causation.

When this study was published

Published in 2015 using data from entering college freshmen.

The bigger picture

The transition to college is a high-risk period for substance-related harm. Understanding that marijuana use amplifies alcohol-related consequences provides actionable information for campus prevention programs that currently may underemphasize polysubstance risks.

Questions still open

• Does marijuana impair judgment during drinking, leading to more consequences? Would addressing marijuana use reduce alcohol-related consequences? Do these polysubstance patterns persist beyond freshman year?

Common questions

Read the original research

Identifying classes of conjoint alcohol and marijuana use in entering freshmen.

Psychology of addictive behaviors : journal of the Society of Psychologists in Addictive Behaviors, 29(3), 620-6

Citation

Haas, Amie L; Wickham, Robert; Macia, Kathryn; Shields, Micah; Macher, Rayna; Schulte, Tilman. (2015). Identifying classes of conjoint alcohol and marijuana use in entering freshmen.. Psychology of addictive behaviors : journal of the Society of Psychologists in Addictive Behaviors, 29(3), 620-6. https://doi.org/10.1037/adb0000089

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