Given the increasing availability of high-potency Δ9-tetrahydrocannabinol (Δ9-THC) products, understanding potency-related effects of Δ9-THC on driving performance is an important public safety issue. This randomized, double-blind, placebo-controlled, within-subjects trial examined the effects of smoked cannabis with varying Δ9-THC concentrations on simulated driving. Adults aged 19–45 who regularly used cannabis and held valid driver’s licenses completed simulated driving tasks after smoking placebo, or cannabis containing low (6.25%/47 mg Δ9-THC), medium (12.5%/94 mg Δ9-THC), or high (22%/165 mg Δ9-THC) Δ9-THC levels. The primary outcome was mean speed (km/h); secondary measures included maximum speed, standard deviation of lateral position (SDLP), reaction time (RT), and subjective ratings of driving ability and intoxication. Mean speed did not differ across conditions. Maximum speed increased under medium (p = 0.006) and high (p = 0.02) potencies versus placebo. SDLP was higher across all Δ9-THC potencies (p < 0.001), and RT was longer under medium and high potencies (p < 0.001). Both SDLP (p < 0.001) and RT (p = 0.023) positively correlated with blood Δ9-THC concentrations. Participants reported poorer driving performance and reduced willingness to drive at higher potencies. Findings demonstrate potency-dependent impairments in simulated driving linked to Δ9-THC concentration, underscoring implications for road safety. Trial registration: clinicaltrials.gov, ID NCT03656029; First posted date: 04/09/2018; URL: https://clinicaltrials.gov/study/NCT03656029.
Brands et al. (2026) studied this question.