Drivers with THC below 5 ng/mL showed no increased crash risk, while alcohol at 0.08% increased risk sixfold
A prospective study of 3,005 injured drivers in British Columbia found no increased crash responsibility with THC below 5 ng/mL and only a non-significant increase at 5+ ng/mL (OR 1.74), compared to a sixfold increase for alcohol above 0.08%.
Quick Facts
What This Study Found
No increased crash risk in drivers with THC <2 or 2-5 ng/mL. At THC >=5 ng/mL, adjusted OR was 1.74 (95% CI 0.59-6.36, non-significant). By contrast, BAC >=0.08% showed OR 6.00 (95% CI 3.87-9.75, p<0.01). Other recreational drugs: OR 1.82. Sedating medications: OR 1.45.
Key Numbers
3,005 injured drivers tested. THC detected in 8.3%. Alcohol in 14.4%. Other drugs in 8.9%. Sedating meds in 19.8%. THC <5 ng/mL: no increased risk. THC >=5 ng/mL: OR 1.74 (NS). BAC >=0.08%: OR 6.00 (p<0.01).
How They Did This
Prospective responsibility analysis of injured drivers at British Columbia trauma centres. Blood from clinical samples tested for broad-spectrum toxicology. Police reports analyzed for crash responsibility. Adjusted for age, sex, and other impairing substances.
Why This Research Matters
This is one of the most rigorous studies comparing cannabis and alcohol driving risk using prospective data and objective blood levels. The findings suggest THC-based per se driving laws (particularly at low thresholds like 2 ng/mL) may be penalizing drivers who are not actually impaired.
The Bigger Picture
Cannabis driving laws in many jurisdictions use THC blood level thresholds modeled on alcohol BAC laws. This study suggests the relationship between THC blood levels and impairment is far weaker than for alcohol, with implications for how driving under the influence of cannabis should be enforced.
What This Study Doesn't Tell Us
Only non-fatally injured drivers included, which may underestimate risk for the most severe crashes. THC blood levels reflect timing of use but not consistent impairment levels across individuals. The 5+ ng/mL group was small, limiting statistical power.
Questions This Raises
- ?Should per se THC limits for driving be reconsidered?
- ?Is a 5 ng/mL threshold too low, too high, or the wrong metric entirely?
- ?Could performance-based impairment testing replace blood level thresholds for cannabis?
Trust & Context
- Key Stat:
- THC <5 ng/mL: no increased risk
- Evidence Grade:
- Rated strong because this is a large prospective study with objective blood level measurements, police crash reports, and appropriate statistical adjustments.
- Study Age:
- Published in 2019, before many jurisdictions finalized cannabis driving laws.
- Original Title:
- Cannabis use as a risk factor for causing motor vehicle crashes: a prospective study.
- Published In:
- Addiction (Abingdon, England), 114(9), 1616-1626 (2019)
- Authors:
- Brubacher, Jeffrey R(5), Chan, Herbert(5), Erdelyi, Shannon(3), Macdonald, Scott, Asbridge, Mark, Mann, Robert E, Eppler, Jeffrey, Lund, Adam, MacPherson, Andrew, Martz, Walter, Schreiber, William E, Brant, Rollin, Purssell, Roy A
- Database ID:
- RTHC-01964
Evidence Hierarchy
Enrolls participants and follows them forward in time.
What do these levels mean? →Frequently Asked Questions
Does cannabis impair driving?
This study found no statistically significant increase in crash responsibility at any THC blood level tested, though drivers with THC above 5 ng/mL showed a trend toward increased risk (OR 1.74). The risk was far lower than for alcohol.
How does cannabis driving risk compare to alcohol?
Alcohol above the legal limit (0.08%) increased crash responsibility sixfold (OR 6.00). The highest THC group showed only OR 1.74, which was not statistically significant.
Are current THC driving limits appropriate?
The findings suggest THC levels below 5 ng/mL are not associated with increased crash risk, raising questions about per se limits set at lower thresholds.
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Cite This Study
https://rethinkthc.com/research/RTHC-01964APA
Brubacher, Jeffrey R; Chan, Herbert; Erdelyi, Shannon; Macdonald, Scott; Asbridge, Mark; Mann, Robert E; Eppler, Jeffrey; Lund, Adam; MacPherson, Andrew; Martz, Walter; Schreiber, William E; Brant, Rollin; Purssell, Roy A. (2019). Cannabis use as a risk factor for causing motor vehicle crashes: a prospective study.. Addiction (Abingdon, England), 114(9), 1616-1626. https://doi.org/10.1111/add.14663
MLA
Brubacher, Jeffrey R, et al. "Cannabis use as a risk factor for causing motor vehicle crashes: a prospective study.." Addiction (Abingdon, 2019. https://doi.org/10.1111/add.14663
RethinkTHC
RethinkTHC Research Database. "Cannabis use as a risk factor for causing motor vehicle cras..." RTHC-01964. Retrieved from https://rethinkthc.com/research/brubacher-2019-cannabis-use-as-a
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.