Do drivers who get into motor vehicle accidents have more THC in their system?

Brubacher JR, Chan H, Erdelyi S, Staples JA, Asbridge M, Mann RE. Cannabis legalization and detection of tetrahydrocannabinol in injured drivers. NEJM 2002;386:148-56. (abstract)

Why was this research conducted?

Because of the cognitive and performance-impairing effects of cannabis, concerns have been expressed that legalization could increase motor vehicle accidents (MVA) associated with cannabis intoxication. To date, studies in different American states have shown mixed effects, so real-world accident data from British Columbia (B.C.) were examined in this study.

What do these articles add?

The study looked at cases in which the driver was injured and had blood drawn as part of treatment at one of four participating B.C. trauma centres. Analysis was conducted if blood was available for the study within six hours of the crash; of 5,699 potential cases, 4,409 (77.4%) were included. Cases consisted of 3,550 prior to cannabis legalization (January 2013-September 2018), 70 during the month legalization was implemented (October 2018), and 789 after legalization (November 2018 to March 2020). There was no significant change pre- versus post-legalization in the percentage who were alcohol impaired (i.e., blood alcohol >0.08%; 9.3% vs. 8.1%; adjusted prevalence ratio=0.98, 95% CI 0.74-1.30). There was significant increase in the proportion whose blood cannabis was:

1) >2 ng/ml (nanogram/milliter), the level of a summary conviction offense (4.7% vs. 8.6%; adj prevalence ratio=2.29, 95% CI 1.52-3.45); and

2) >5 ng/ml, the level of a summary or indictable offense (1.1% vs. 3.5%, adj prevalence ratio = 2.05, 95% CVI 1.00-4.18).

Summary offenses are less serious and the punishments are less severe compared to indictable offenses. The distinction is similar to misdemeanor and felony in the American system.

Is there anything else I should know?

As the authors note, the presence of THC does not necessarily mean it causes or increases the risk of collision. The database contained a select sub-set of all MVAs, as it excluded fatalities, those with minor or no injuries, and those not transported to one of the study centres. No information is provided on the mode of cannabis administration or prior cannabis use, even though these factors can affect blood levels. The prevalence of cannabis use in B.C. (both before and after legalization) tends to be higher than the national average, so results may not be applicable to other jurisdictions. These considerations nonetheless suggest that cannabis is more frequently associated with motor vehicle accidents following legalization.