High-'n'-dry? A comparison of cannabis and alcohol use in drivers presenting to hospital after a vehicular collision.

DESIGN: This was a prospective observational study. BACKGROUND AND AIMS: The characteristics of cannabis-involved motor vehicle collisions are poorly understood. This study of injured drivers identifies demographic and collision characteristics associated with high tetrahydrocannabinol (THC) concentrations. SETTING: The study was conducted in 15 Canadian trauma centres between January 2018 and December 2021. CASES: The cases (n = 6956) comprised injured drivers who required blood testing as part of routine trauma care. MEASUREMENTS: We quantified whole blood THC and blood alcohol concentration (BAC) and recorded driver sex, age and postal code, time of crash, crash type and injury severity. We defined three driver groups: high THC (THC ≥ 5 ng/ml and BAC = 0), high alcohol (BAC ≥ 0.08% and THC = 0) and THC/BAC-negative (THC = 0 = BAC). We used logistic regression techniques to identify factors associated with group membership. FINDINGS: Most injured drivers (70.2%) were THC/BAC-negative; 1274 (18.3%) had THC > 0, including 186 (2.7%) in the high THC group; 1161 (16.7%) had BAC > 0, including 606 (8.7%) in the high BAC group. Males and drivers aged less than 45 years had higher adjusted odds of being in the high THC group (versus the THC/BAC-negative group). Importantly, 4.6% of drivers aged less than 19 years had THC ≥ 5 ng/ml, and drivers aged less than 19 years had higher unadjusted odds of being in the high THC group than drivers aged 45-54 years. Males, drivers aged 19-44 years, rural drivers, seriously injured drivers and drivers injured in single-vehicle, night-time or weekend collisions had higher adjusted odds ratios (aORs) for being in the high alcohol group (versus THC/BAC-negative). Drivers aged less than 35 or more than 65 years and drivers involved in multi-vehicle, daytime or weekday collisions had higher adjusted odds for being in the high THC group (versus the high BAC group). CONCLUSIONS: In Canada, risk factors for cannabis-related motor vehicle collisions appear to differ from those for alcohol-related motor vehicle collisions. The collision factors associated with alcohol (single-vehicle, night-time, weekend, rural, serious injury) are not associated with cannabis-related collisions. Demographic factors (young drivers, male drivers) are associated with both alcohol and cannabis-related collisions, but are more strongly associated with cannabis-related collisions.