Influence of cannabis and alcohol on motor vehicle injury severity in Canadian trauma centres: a prospective study

Alcohol and delta-9-tetrahydrocannabinol (THC) (main impairing ingredient of cannabis) are both crash contributors that interfere with motor vehicle operation. However, the relationship between drug concentration and crash injury severity is unclear for either drug. We aim to clarify the relationship between blood alcohol concentration (BAC) and crash injury severity, based on healthcare system utilisation, with and without THC.

The National Drug Driving Study is an ongoing prospective study involving 17 Canadian trauma centres. Eligible subjects included drivers aged 16+ who visited a participating trauma centre and had blood drawn as part of routine care within 6 hours of a crash. Deidentified blood samples were tested for alcohol and THC using gas chromatography-flame ionisation detection and liquid chromatography/tandem mass spectrometry. Study outcomes included admission to hospital and admitted patients' length of hospital stay.

10 322 injured drivers visited a participating trauma centre between 2018 and 2023. 1649 (16.0%), 1716 (16.6%) and 463 (4.5%) drivers had detectable levels of alcohol, THC or both, respectively. Compared with sober drivers (BAC=0), drivers with 0%

Alcohol increases hospital admissions after crashes but does not have a dose-response relationship with admission or length of stay. THC does not moderate this relationship.