THC Edibles Impair Driving Even Below Legal Blood THC Thresholds: JAMA
Canada: A randomized crossover clinical trial has revealed that delta-9-tetrahydrocannabinol (THC)-containing edibles impaired simulated driving performance in a dose-dependent manner. Importantly, impairment occurred even when blood THC concentrations were below commonly used per se legal thresholds for roadside enforcement, suggesting that THC blood levels alone may not reliably indicate driving fitness.
The study, published in JAMA Network Open by Bernard Le Foll, MD, PhD, Institute for Mental Health Policy Research, Centre for Addiction and Mental Health, Toronto, Ontario, Canada, and colleagues, evaluated whether commercially available cannabis edibles affect driving performance according to the amount of THC consumed.
A double-blind, placebo-controlled crossover trial conducted at a tertiary academic hospital in Toronto from September 2024 to April 2025 enrolled healthy adults aged 19–45 years who used cannabis at least weekly and cannabis edibles at least monthly.
Participants received soft-chew gummies containing 0, 2, 10, or 20 mg of delta-9-tetrahydrocannabinol (THC). The primary outcome was standard deviation of lateral position (SDLP), a measure of lane-control consistency during simulated driving. Researchers also assessed driving-speed variability, reaction time, willingness to drive, perceived driving ability, and blood THC and metabolite concentrations.
Of 85 individuals assessed, 52 were enrolled. Following 10 withdrawals before dosing and 2 exclusions, 40 participants were included in the final analysis. Participants had a mean age of 30.4 years, with 20 women and 20 men.
The trial revealed the following findings:
- The 20-mg THC dose increased SDLP by 3.3 cm, while the 10-mg dose increased SDLP by 2.0 cm compared with placebo; both changes were statistically significant.
- The 2-mg THC dose increased SDLP by 0.7 cm, but the difference was not statistically significant.
- The 20-mg dose prolonged reaction time by 0.034 seconds and significantly increased driving-speed variability, indicating poorer vehicle control and slower responses.
- Participants reported reduced willingness to drive and lower perceived driving ability as THC doses increased, with the largest declines observed after 20 mg.
- Peak blood THC concentrations reached approximately 3.4 ng/mL after 20 mg and 1.6 ng/mL after 10 mg, despite the observed driving impairment.
- THC-related driving impairment occurred at blood concentrations near or below commonly used per se legal thresholds of 2 or 5 ng/mL, raising questions about relying on THC blood levels alone to determine driving fitness.
The study had several limitations, including a relatively simple rural driving simulation that may not reflect complex urban traffic. It assessed only lane control, speed variability, and reaction time, leaving other driving aspects such as hazard perception and braking unexamined. Additionally, the duration of impairment between 5 and 24 hours after THC ingestion remains unclear.
Overall, the findings indicate that THC edibles can impair driving performance in a dose-related manner, particularly at 10-mg and 20-mg doses, while measurable impairment may occur at blood THC concentrations below commonly used roadside enforcement thresholds.
Reference:
Le Foll B, Matheson J, Antwi P, et al. Dose-Dependent Effects of Cannabis Edibles on Simulated Driving Performance: A Randomized Clinical Trial. JAMA Netw Open. 2026;9(8):e2631306. doi:10.1001/jamanetworkopen.2026.31306
Next Story