As cannabis edibles become increasingly mainstream, a critical blind spot has persisted in road safety science: how much do they actually impair driving, and for how long? This question carries legal weight given that current per se THC blood thresholds — typically 2 or 5 ng/mL — were calibrated largely around smoked cannabis, not oral formulations with their slower, more variable absorption profiles.
This randomized, double-blind, placebo-controlled crossover trial enrolled 52 regular cannabis users (aged 19–45) at a Toronto academic hospital between September 2024 and April 2025. Participants received commercially available soft-chew gummies delivering 0, 2, 10, or 20 mg of delta-9-THC across separate sessions. Driving impairment was assessed using standard deviation of lateral position (SDLP) — a validated surrogate for lane-keeping — on a driving simulator at both two and five hours post-ingestion. The 20 mg dose produced statistically significant SDLP increases at both time points, while lower doses showed more modest effects. Notably, self-reported willingness to drive and perceived ability did not reliably track objective performance decrements, a dissociation that compounds public safety concerns.
This trial addresses a genuine evidence gap, and its crossover design with an intention-to-treat analysis strengthens internal validity. However, the sample is limited to experienced, regular cannabis users — a population with established tolerance — meaning impairment in occasional users could be substantially greater. Simulator performance, while validated, does not fully replicate real-world driving complexity or consequences. Critically, the data may undermine the legal defensibility of fixed blood THC thresholds for edibles: oral THC pharmacokinetics produce peak impairment that often lags behind peak blood concentration, meaning a driver could be legally below the per se limit yet functionally impaired. This finding is incrementally significant within road safety science but potentially paradigm-shifting for cannabis impairment legislation.