Combining alcohol and cannabis increased driving impairment beyond either drug alone | Research

The takeaway

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.

Cannabis and alcohol users, driving safety researchers, and public health communicators.

Participants were unaware of their increased combined impairment

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

Well-designed double-blind, placebo-controlled crossover RCT with simulated driving, though real-world generalizability is limited.

When this study was published

Published in 2022.

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.

Questions still open

At what alcohol and cannabis doses does the combined impairment become dangerous? Would awareness of combined impairment change behavior if communicated effectively?

Common 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.

Read the original research

Combined effect of alcohol and cannabis on simulated driving.

Psychopharmacology, 239(5), 1263-1277

View on PubMed ↗Publisher’s paper ↗

Citation

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