Driving by frequent cannabis users 'the morning after' last use of smoked cannabis: an observational driving simulator study.

Zakala, C; Zhao, S; Battistuzzi, A; Nette, A; Matheson, J; Le Foll, B; Brands, B; Wickens, CM; Kaduri, P; Hasan, O; Wang, W; Chen, S; Di Ciano, P · 2026 · PubMed Central

DOI: 10.1186/s42238-026-00416-w

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Summary

This observational study investigated whether frequent cannabis users exhibit residual driving impairment 12–15 hours after their last use of smoked cannabis, and whether such impairment correlates with delta-9-tetrahydrocannabinol (THC) concentrations in blood and oral fluid. The research was motivated by conflicting evidence regarding the duration of cannabis-induced driving deficits and the uncertain relationship between biomarker levels and driving performance, particularly in the context of increasing cannabis legalization. The study employed a parallel-group design involving 130 participants: 65 frequent cannabis users (smoking ≥4 times/week) and 65 matched non-users. Participants drove a simulator 12–15 hours after the cannabis group’s last use. Driving performance was assessed via standard deviation of lateral position (SDLP), speed, following distance, and brake latency under both standard and dual-task conditions. Blood and oral fluid samples were analyzed for THC, cannabidiol (CBD), and metabolites. Statistical analyses adjusted for multiple comparisons using the Benjamini-Hochberg procedure. Results indicated no significant impairment in driving performance for the cannabis group compared to controls. While unadjusted analyses showed small differences in SDLP favoring the cannabis group, these were not significant after correction for multiple comparisons. Measures of speed, following distance, and brake latency were unaffected. Crucially, neither blood nor oral fluid concentrations of THC, CBD, or metabolites significantly correlated with any driving metrics after correction. Mean blood THC levels exceeded 2 ng/mL, a threshold often cited in legal contexts. However, trends suggested a potential relationship between driving performance and the psychoactive metabolite 11-hydroxy-THC. Subjectively, cannabis users reported higher feelings of intoxication and greater willingness to drive, though these differences also lost significance after statistical correction. The study concludes that frequent cannabis users do not demonstrate significant objective driving impairment 12–15 hours after last use, despite detectable THC levels. The findings challenge the utility of blood and oral fluid THC concentrations as accurate correlates of driving behavior in this timeframe. The authors suggest that large-scale studies are needed to determine if less frequent users are impaired the morning after use and to investigate whether these findings apply to other routes of administration.

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• cannabis
• polydrug
• illicit drugs
• alcohol

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• Methodological Resource: validation psychometrics, tool software