A 40-volunteer Toronto trial found 10- and 20-milligram THC gummies impaired simulated driving at two and five hours after use, despite blood THC levels often staying under legal roadside thresholds.
At 20 milligrams, drivers showed slower reaction times, worse lane control and less consistent speed; peak blood THC averaged 3.4 ng/mL, below the commonly used 5-ng/mL cutoff.
Researchers said edibles create a mismatch between impairment and blood-test results because oral THC is absorbed more slowly than smoked cannabis, which in a prior study produced similar impairment at 30.3 ng/mL.
The study used regular cannabis users and a simulator rather than real roads, though residual effects were minimal after 24 hours even as subjective intoxication had faded.
An accompanying commentary said the findings complicate impaired-driving enforcement in Canada and some U.S. jurisdictions, increasing pressure for better roadside tools than standard blood THC tests.