College freshmen who used both marijuana and alcohol had more blackouts, injuries, and DUIs than drinkers who didn't use marijuana
Among 772 college freshmen, moderate drinkers who also used marijuana reported more blackouts, physical injuries, DUI, and drinking-to-get-drunk compared to moderate drinkers who did not use marijuana.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- Marijuana + moderate drinking = more consequences than moderate drinking alone
- Evidence Grade:
- Cross-sectional latent profile analysis with a reasonably large freshman sample. Cannot establish causation.
- Study Age:
- Published in 2015 using data from entering college freshmen.
- Original Title:
- Identifying classes of conjoint alcohol and marijuana use in entering freshmen.
- Published In:
- Psychology of addictive behaviors : journal of the Society of Psychologists in Addictive Behaviors, 29(3), 620-6 (2015)
- Authors:
- Haas, Amie L, Wickham, Robert, Macia, Kathryn, Shields, Micah, Macher, Rayna, Schulte, Tilman
- Database ID:
- RTHC-00972
Evidence Hierarchy
A snapshot of a population at one point in time.
What do these levels mean? →Frequently Asked Questions
Is combining marijuana and alcohol more dangerous?
In this study, college freshmen who used both marijuana and alcohol at moderate levels experienced significantly more blackouts, injuries, and DUIs compared to moderate drinkers who did not use marijuana, even at similar drinking levels.
How common is combined use among college freshmen?
In this sample, 36% of freshmen (22% moderate drinkers + 14% heavy drinkers) used both alcohol and marijuana, suggesting polysubstance use is common from the start of college.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-00972APA
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
MLA
Haas, Amie L, et al. "Identifying classes of conjoint alcohol and marijuana use in entering freshmen.." Psychology of addictive behaviors : journal of the Society of Psychologists in Addictive Behaviors, 2015. https://doi.org/10.1037/adb0000089
RethinkTHC
RethinkTHC Research Database. "Identifying classes of conjoint alcohol and marijuana use in..." RTHC-00972. Retrieved from https://rethinkthc.com/research/haas-2015-identifying-classes-of-conjoint
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.