Potency-related effects of smoked cannabis on simulated driving performance: a randomized, controlled crossover trial.
Bruna Brands, Adam Zaweel, Madison Wright, Patricia Di Ciano, Christine M Wickens, Justin Matheson, Andrew Fares, Omer S M Hasan, Marcos Sanches, Beth Sproule, Marilyn A Huestis, Timothy L Brown, Bernard Le Foll
Scientific Reports March 10, 2026 DOI: 10.1038/s41598-026-43045-2 (opens in new tab) via PubMed
Summary
AI-generated from the abstractSmoked cannabis containing varying levels of Δ9-THC produced potency-dependent impairments in simulated driving. In a randomized, double-blind, placebo-controlled trial, adults who regularly used cannabis and held valid driver's licenses completed driving tasks after smoking placebo or cannabis with low (6.25%/47 mg), medium (12.5%/94 mg), or high (22%/165 mg) Δ9-THC. Mean speed did not differ across conditions, but maximum speed increased under medium and high potencies versus placebo. Standard deviation of lateral position (SDLP) was higher across all potencies, and reaction time (RT) was longer under medium and high potencies. Both SDLP and RT correlated positively with blood Δ9-THC concentrations. Participants reported poorer driving performance and reduced willingness to drive at higher potencies.
Study at a glance
| Characteristics | Randomized controlled trial Placebo-controlled Double-blind Peer reviewed |
|---|---|
| Population | Adults aged 19–45 who regularly used cannabis and held valid driver's licenses |
| Intervention | Smoked cannabis |
| Dose | low (6.25%/47 mg Δ9-THC), medium (12.5%/94 mg Δ9-THC), high (22%/165 mg Δ9-THC) |
| Topics | Cannabis |
| Keywords | Ad libitum Cannabis potency Simulated driving |
| Registration | NCT03656029 |
| Key finding | Cannabis containing medium or high Δ9-THC increased maximum speed, and all potencies increased SDLP, while medium and high potencies lengthened reaction time during simulated driving, indicating potency-dependent impairment. |
Abstract
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 .