Alcohol Increased Hospital Admissions After Crashes, but THC Did Not Add to the Effect
In over 10,000 injured drivers at Canadian trauma centers, alcohol increased crash-related hospital admissions but THC did not modify this relationship.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?Does THC increase crash risk even if it doesn't increase injury severity?
- ?Would results differ with more precise measures of THC impairment?
Trust & Context
- Key Stat:
- Evidence Grade:
- Large multi-center prospective design with toxicological verification, but inability to measure impairment level and selection bias from trauma center population limit to moderate.
- Study Age:
- Data collected 2018-2023 across 17 Canadian trauma centers.
- Original Title:
- Influence of cannabis and alcohol on motor vehicle injury severity in Canadian trauma centres: a prospective study.
- Published In:
- Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention (2025)
- Authors:
- Simmons, Sarah M(2), Donoghue, Madison, Erdelyi, Shannon(3), 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
- Database ID:
- RTHC-07664
Evidence Hierarchy
Watches what happens naturally without intervening.
What do these levels mean? →Frequently Asked Questions
Does this mean THC doesn't make driving more dangerous?
No. This study measured injury severity after crashes, not whether THC increased the likelihood of crashing in the first place. THC may still increase crash risk.
Did combining alcohol and THC make injuries worse?
No. THC did not modify alcohol's effect on hospital admission or length of stay. Alcohol alone was associated with increased admissions.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-07664APA
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
MLA
Simmons, Sarah M, et al. "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, 2025. https://doi.org/10.1136/ip-2025-045642
RethinkTHC
RethinkTHC Research Database. "Influence of cannabis and alcohol on motor vehicle injury se..." RTHC-07664. Retrieved from https://rethinkthc.com/research/simmons-2025-influence-of-cannabis-and
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.