Cannabis Edibles Impair Driving Below Legal THC Thresholds, Trial Finds

Commercially available cannabis edibles dose-dependently impaired simulated driving performance, even at blood tetrahydrocannabinol (THC) concentrations at or below the thresholds many jurisdictions use to enforce cannabis-impaired driving laws.¹ The findings, published in JAMA Network Open, add to evidence that current per se thresholds—blood THC levels that trigger automatic legal impairment findings commonly set at 2 or 5 ng/mL—may not reliably capture impairment from edible cannabis products, whose pharmacokinetics differ substantially from inhaled forms.

“This placebo-controlled, crossover randomized clinical trial found significant evidence of dose-related impairment of driving simulator performance 2 and 5 (but not 24) hours after ingestion of THC edibles among adults who used cannabis regularly,” wrote the researchers of the study. “We found decreased lane control, increased speed variability, and increased RT [reaction time], especially at the highest dose (20 mg), with minimal evidence of changes in driving performance at the low dose (2 mg).”

Researchers at the Centre for Addiction and Mental Health in Toronto included 40 healthy adults (20 female, 20 male; mean age, 30.4 years) who reported at least weekly cannabis use and monthly edible use. Participants completed 4 dosing sessions in randomized order, receiving soft-chew gummies containing 0, 2, 10, or 20 mg of delta-9-THC. Driving simulator performance, self-reported driving willingness and perceived ability, and whole-blood THC levels were assessed 2, 5, and 24 hours after exposure. The primary outcome was standard deviation of lateral position (SDLP), a validated marker of lane control.

Dose-Dependent Impairment in Lane Control and Reaction Time

Relative to placebo, SDLP was significantly higher at the 20-mg dose (difference in means, 3.3 cm; 95% CI, 2.4-4.3; P < .001) and the 10-mg dose (2.0 cm; 95% CI, 1.0-2.9; P < .001), but not at 2 mg (0.7 cm; 95% CI, −0.3 to 1.6; P = .16). Reaction time was significantly slower at 20 mg (difference in means, 0.034 seconds; 95% CI, 0.018-0.050; P < .001), which was the only driving measure that differed from placebo at the lowest dose tested. The standard deviation of speed was significantly higher at 20 mg (0.469 km/h; 95% CI, 0.154-0.784; P = .004). Self-reported willingness to drive and perceived driving ability declined in a dose-dependent manner across all 3 active doses.

Blood THC Levels Stayed Below Common Per Se Limits

Peak whole-blood THC concentrations were 3.4 ng/mL at the 20-mg dose and 1.6 ng/mL at the 10-mg dose—both at or below the 2 to 5 ng/mL thresholds used in Canada and several US states. The authors noted that this pattern differed from a related trial of smoked cannabis by the same research group, in which comparable driving impairment occurred at blood THC levels nearly 10-fold higher (30.3 ng/mL), reflecting the slower absorption and lower peak concentrations characteristic of oral THC administration.

Minimal residual impairment was observed 24 hours after exposure; only maximum driving speed remained elevated at the 10- and 20-mg doses, and no differences emerged in subjective intoxication ratings at that time point.

Cannabis Edibles Challenge Roadside THC Enforcement Standards

The US has no nationwide blood-THC threshold for impaired driving, as of May 2026, and roadside tools are unable to reliably distinguish current impairment from recent past use.² Because edibles produce measurable impairment at blood concentrations below widely used legal thresholds, they may present a unique challenge to the enforcement of per se legislation.¹

“Objective measures of driving simulator performance aligned with self-reported data suggesting that participants were less willing to drive and perceived lower driving ability in a dose-dependent manner, even at the low dose,” wrote the researchers. “Whole-blood THC levels were below the commonly used 5 ng/mL per se threshold in all dose conditions, suggesting that THC edibles may present unique challenges for roadside enforcement.”

References

1. 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
2. Harwood S. Cannabis edibles and alcohol impair drivers beyond legal drunk limit: tests cannot detect it. Tech Times. May 31, 2026. Accessed September 2, 2026. https://www.techtimes.com/articles/317471/20260531/cannabis-edibles-alcohol-impair-drivers-beyond-legal-drunk-limit-tests-cannot-detect-it.htm