Cannabis use and driving-related performance in young recreational users: a within-subject randomized clinical trial

Background: With the legalization of cannabis in Canada, young adults, who are already at risk of automobile crashes, may increase their use of cannabis, which may further increase the risk of crashes. We examined the effects of inhaled cannabis on driving-related performance in healthy 18- to 24-year-old recreational cannabis users.

Methods: In this within-subject randomized study, participants completed tests in the no-cannabis state and at 1, 3 and 5 hours after inhalation of a standard 100-mg dose of cannabis. We then measured performance (in useful-field-of-view and driving-simulation tests) and self-reported perceptions (driving ability and safety, cannabis effects). Repeated-measures analysis of variance (for cannabis effects on continuous performance measures), Cochran Q tests (for performance-related crash risk and binary complex simulator task scores) and correlational analyses (for self-reported perceptions relative to performance) were employed.

Results: Forty-five participants completed all 180 testing sessions. Significant effects of cannabis (relative to no cannabis) were noted on complex useful-field-of-view tasks at 3 hours (complex divided-attention task: 70 ± 24 ms v. 37 ± 12 ms, 95% confidence intervals [CIs] 28–114 ms v. 29–45 ms, t = −2.98, df = 41, p = 0.005; complex selective-attention task: 102 ± 66 ms v. 64 ± 18 ms, 95% CIs 60–144 ms v. 53–75 ms, t = −2.42, df = 41, p = 0.02) and 5 hours (complex selective-attention task: 82 ± 29 ms v. 61 ± 19 ms, 95% CIs 62–100 ms v. 48–75 ms, t = −2.32, df = 41, p = 0.03) after cannabis use when the tasks were novel (performed in a cannabis state at the first session). Participants were significantly more likely to be classified as having a high crash risk (on the basis of simulator tasks) after cannabis use (χ² = 13.23, df = 1, p < 0.001, odds ratio 4.31, 95% CI 0.41–45.2) and reported significantly lower perceived driving ability and safety after cannabis use relative to non-use.

Interpretation: Among young recreational cannabis users, a 100-mg dose of cannabis by inhalation had no effect on simple driving-related tasks, but there was significant impairment on complex tasks, especially when these were novel. These effects, along with lower self-perceived driving ability and safety, lasted up to 5 hours after use.

Trial registration: The trial was registered with Health Canada (NOL [No Objection Letter] no. 215101).