Research study · Scientific Reports · 2026
Potency-related effects of smoked cannabis on simulated driving performance: a randomized, controlled crossover trial
Verified on PubMed Oxford CEBM Level 1B: Randomized Controlled Trial
A 2026 randomized controlled trial in Scientific Reports tested cannabis in driving performance. Stronger cannabis made driving worse, but not in every way. This study suggests that higher THC concentrations in smoked cannabis lead to greater impairment in simulated driving tasks, which may inform how clinicians discuss potency risks with patients who drive.
In plain language
Study overview
Researchers in Toronto studied adults who use cannabis regularly and hold driver's licences. Each person sat in a driving simulator after smoking cannabis of different strengths, or a dummy product with no active ingredient, on separate days.
Findings
Stronger cannabis made driving worse, but not in every way. Steering control and reaction time got worse, and people felt less confident about driving.
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The deeper readScientific analysis
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.
The paper
Citation
Brands B, Zaweel A, Wright M, Di Ciano P, Wickens CM, Matheson J, et al. Potency-related effects of smoked cannabis on simulated driving performance: a randomized, controlled crossover trial. Scientific Reports (2026). PubMed 41807521 · DOI
Limitations the authors noted
Primary outcome (mean speed) unaffected, so the primary hypothesis was not supported. Switch from fixed to ad libitum smoking during the study. Δ9-THC content of low and medium cigarettes not verified. Small sample, with 35 completers and 13 early discontinuations; p-values unadjusted for multiple comparisons. Sample of regular users may not generalize to infrequent users or people with cannabis use disorder. Two blood assay methods used (GC-MS and LC-MS/MS). Driving assessed only at 30 and 90 minutes; effects beyond 90 minutes unknown. Simulator study, not on-road driving.
In practice
Why this matters
This study suggests that higher THC concentrations in smoked cannabis lead to greater impairment in simulated driving tasks, which may inform how clinicians discuss potency risks with patients who drive.
Frequently asked questions
Clinical Questions & Answers
What were the primary clinical findings regarding Cannabis and Cannabis use?
Stronger cannabis made driving worse, but not in every way. Steering control and reaction time got worse, and people felt less confident about driving.
What study limitations or clinical safeguards should patients keep in mind?
Primary outcome (mean speed) unaffected, so the primary hypothesis was not supported. Switch from fixed to ad libitum smoking during the study. Δ9-THC content of low and medium cigarettes not verified.
Browse related research
Condition: Cannabis use Condition: Driving performance Topic: Cannabis Topic: Marijuana smoking Topic: THC
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Reviewed by Dr. Caplan on October 20, 2026.
This page is general education about a published study. It is not medical advice and does not create a doctor-patient relationship. For care specific to you, book a consultation.
