Combining alcohol and cannabis increased driving impairment beyond either drug alone
In a controlled driving simulation trial, combining alcohol and cannabis increased lane weaving and reaction time more than either substance alone, and participants were unaware of their heightened impairment.
Quick Facts
What This Study Found
The combination of alcohol (target BAC 0.08%) and cannabis (12.5% THC) significantly increased standard deviation of lateral position (weaving) compared to placebo and to either drug alone. Reaction time also increased. Subjective effects were greater than with either drug alone, particularly later in the session. Critically, participants seemed unaware of their increased impairment on a driving ability questionnaire.
Key Numbers
Cannabis: 12.5% THC smoked. Alcohol: target BrAC 0.08%. Combined condition significantly increased weaving vs placebo (p<0.001), vs alcohol alone (p=0.029), and vs cannabis alone (p=0.032).
How They Did This
Within-subjects, double-blind, double-dummy, placebo-controlled randomized clinical trial with 19-29-year-old cannabis users (1-7 days/week). Four sessions: placebo/placebo, alcohol/placebo, placebo/cannabis, alcohol/cannabis. Simulated driving assessed with single and dual tasks.
Why This Research Matters
Alcohol and cannabis are the two most commonly detected drugs in seriously and fatally injured drivers. Understanding their combined effects is critical for road safety messaging.
The Bigger Picture
The finding that users were unaware of their combined impairment is particularly dangerous, as it means people who combine these substances may feel capable of driving when they are significantly impaired.
What This Study Doesn't Tell Us
Simulated driving may not fully replicate real-world conditions. Young, experienced cannabis users may not represent all populations. Single dose level tested for each substance.
Questions This Raises
- ?At what alcohol and cannabis doses does the combined impairment become dangerous?
- ?Would awareness of combined impairment change behavior if communicated effectively?
Trust & Context
- Key Stat:
- Participants were unaware of their increased combined impairment
- Evidence Grade:
- Well-designed double-blind, placebo-controlled crossover RCT with simulated driving, though real-world generalizability is limited.
- Study Age:
- Published in 2022.
- Original Title:
- Combined effect of alcohol and cannabis on simulated driving.
- Published In:
- Psychopharmacology, 239(5), 1263-1277 (2022)
- Authors:
- Fares, Andrew(3), Wickens, Christine M(9), Mann, Robert E(11), Di Ciano, Patricia, Wright, Madison, Matheson, Justin, Hasan, Omer S M, Rehm, Jurgen, George, Tony P, Samokhvalov, Andriy V, Shuper, Paul A, Huestis, Marilyn A, Stoduto, Gina, Brown, Timothy, Stefan, Cristiana, Rubin-Kahana, Dafna Sara, Le Foll, Bernard, Brands, Bruna
- Database ID:
- RTHC-03831
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
Was the combined effect additive?
The results suggested a potentially additive effect on lane weaving and subjective impairment, with the combination producing significantly more impairment than either substance alone.
Did people know they were more impaired?
No. A driving ability questionnaire showed that participants seemed unaware of their increased level of impairment when using both substances together, which has serious implications for driving decisions.
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Cite This Study
https://rethinkthc.com/research/RTHC-03831APA
Fares, Andrew; Wickens, Christine M; Mann, Robert E; Di Ciano, Patricia; Wright, Madison; Matheson, Justin; Hasan, Omer S M; Rehm, Jurgen; George, Tony P; Samokhvalov, Andriy V; Shuper, Paul A; Huestis, Marilyn A; Stoduto, Gina; Brown, Timothy; Stefan, Cristiana; Rubin-Kahana, Dafna Sara; Le Foll, Bernard; Brands, Bruna. (2022). Combined effect of alcohol and cannabis on simulated driving.. Psychopharmacology, 239(5), 1263-1277. https://doi.org/10.1007/s00213-021-05773-3
MLA
Fares, Andrew, et al. "Combined effect of alcohol and cannabis on simulated driving.." Psychopharmacology, 2022. https://doi.org/10.1007/s00213-021-05773-3
RethinkTHC
RethinkTHC Research Database. "Combined effect of alcohol and cannabis on simulated driving..." RTHC-03831. Retrieved from https://rethinkthc.com/research/fares-2022-combined-effect-of-alcohol
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.