REVIEW OF: Brubacher JR et al, N Engl J Med 2022;386(2):148–156

STUDY TYPE: Prospective study

As recreational cannabis use has grown in popularity and accessibility, so have concerns about the risks of cannabis-impaired driving. While we know that cannabis can impair driving performance in a controlled setting (see CATR Jan/Feb/Mar 2023), what about in the real world?

Canada’s nationwide legalization of recreational cannabis in 2018 provided researchers with an opportunity to answer this question. They identified 4,339 drivers treated for moderate injuries from motor vehicle collisions who submitted blood samples as part of their care between 2013 and 2020. Authors divided the drivers into a pre-legalization group (January 2013–September 2018) and a post-legalization group (November 2018–March 2020) and compared THC and alcohol serum levels between them.

The results were eye-opening. The percentage of drivers with THC in their blood nearly doubled after legalization (from 9.2% to 17.9%). The percentage of drivers with a THC level at or above the Canadian legal limit of 2 ng/mL increased from 3.8% to 8.6%, while the percentage of drivers testing at ≥5 ng/mL increased from 1.1% to 3.5%. There was no concomitant increase in the prevalence of drivers testing positive for alcohol. Interestingly, the increase was most pronounced among older drivers (over the age of 50) and male drivers.

There were several limitations to this study. It only included moderately injured drivers, so those with minor injuries or fatal accidents were not accounted for. Additionally, there was variability in the time between the collision and the serum sample, which could impact the accuracy of the THC levels. And although a causative link between cannabis prevalence and injury is plausible, the data in this study remain purely associative.

CARLAT TAKE

This prospective study found that the proportion of moderately injured drivers testing positive for THC increased after the passage of a recreational cannabis law. Patients are aware of the dangers while driving under the influence of alcohol, but this study suggests that you should advise your patients to avoid driving after using cannabis as well.