Study: Combined Ingestion of Cannabis and Alcohol Possesses Additive Effects on Driving Performance

Toronto, Canada: The simultaneous ingestion of inhaled cannabis and alcohol results in more significant changes in simulated driving performance than does the consumption of either substance alone, according to data published in the journal Psychopharmacology

A team of investigators affiliated with the University of Toronto assessed simulated driving performance in 28 subjects following their use of placebo alcohol and placebo cannabis (<0.1 percent THC), alcohol and placebo cannabis, placebo alcohol and active cannabis, and alcohol and active cannabis.

Subjects’ performance following the ingestion of both alcohol and cannabis together “was significantly greater than the effect of each drug by itself,” authors reported – a finding that is consistent with prior research. In particular, simultaneous exposure to both substances appeared to possess additive adverse effects on subjects’ ability to maintain lateral positioning and speed. Subjects’ reaction time was also significantly influenced by the dual administration of cannabis and alcohol. 

Authors concluded, “Previous studies provided some evidence for an additive effect of the combination of cannabis and alcohol on driving measures and the current results … support these findings.”

Full text of the study, “Combined effect of alcohol and cannabis on simulated driving,” appears in Psychopharmacology. Further information is available from the NORML fact sheet, “Marijuana and Psychomotor Performance.