In over 10,000 injured drivers at Canadian trauma centers, alcohol increased crash-related hospital admissions but THC did not modify this relationship.
traffic safety researchers, cannabis policy analysts, emergency medicine professionals
What the researchers found
Drivers with any detectable alcohol had increased odds of hospital admission (aOR 1.36-1.69), but THC did not modify the relationship between alcohol and admission. Neither alcohol nor THC predicted length of hospital stay after admission. 16% of drivers had detectable alcohol, 16.6% had detectable THC, and 4.5% had both.
Why it matters
With cannabis legalization, understanding whether THC compounds alcohol's effect on crash injury severity is critical for policy. This large prospective study with toxicological verification provides some of the strongest data on this question.
The numbers in context
10,322 injured drivers from 17 trauma centers. 16.0% had detectable alcohol, 16.6% had detectable THC, 4.5% had both. BAC >0 to <0.08%: aOR 1.69 for admission. BAC >=0.08%: aOR 1.36 for admission. THC did not modify alcohol's effect on admission or predict length of stay.
How the study worked
Prospective study across 17 Canadian trauma centers (2018-2023) as part of the National Drug Driving Study. Blood samples from 10,322 injured drivers aged 16+ were tested for alcohol (GC-FID) and THC (LC-MS/MS) within 6 hours of the crash. Outcomes were hospital admission and length of stay.
What this study cannot tell us
Measures injury severity after crashes, not crash risk. Blood THC levels were measured but do not reliably indicate impairment level. Selection bias from only including drivers who reached trauma centers. No data on cannabis tolerance or frequency of use.
How to read the evidence
Large multi-center prospective design with toxicological verification, but inability to measure impairment level and selection bias from trauma center population limit to moderate.
When this study was published
Data collected 2018-2023 across 17 Canadian trauma centers.
The bigger picture
The finding that THC did not independently predict injury severity or modify alcohol's effect is notable, but does not mean THC-impaired driving is safe. This study measured injury severity, not crash risk itself.
Questions still open
- Does THC increase crash risk even if it doesn't increase injury severity?
- Would results differ with more precise measures of THC impairment?
Common questions
Does this mean THC doesn't make driving more dangerous?
Did combining alcohol and THC make injuries worse?
Read the original research
Influence of cannabis and alcohol on motor vehicle injury severity in Canadian trauma centres: a prospective study.
Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention
Citation
Simmons, Sarah M; Donoghue, Madison; Erdelyi, Shannon; Chan, Herbert; Vaillancourt, Christian; Atkinson, Paul; Besserer, Floyd; Clarke, David B; Davis, Phil; Daoust, Raoul; Émond, Marcel; Eppler, Jeffrey; Lee, Jacques S; MacPherson, Andrew; Magee, Kirk; Mercier, Eric; Ohle, Robert; Parsons, Michael; Rao, Jagadish; Rowe, Brian H; Taylor, John; Wishart, Ian; Brubacher, Jeffrey R. (2025). Influence of cannabis and alcohol on motor vehicle injury severity in Canadian trauma centres: a prospective study.. Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention. https://doi.org/10.1136/ip-2025-045642