A study published in JAMA Network Open indicates that consuming cannabis edibles can significantly impair driving performance and reaction times, even when blood tests show THC levels below the legal limit. This research highlights challenges for impaired-driving enforcement in Canada and some U.S. jurisdictions. Unlike smoked marijuana, which rapidly increases THC levels in the bloodstream, edibles are absorbed more slowly and can cause impairment at lower blood THC concentrations.
Researchers from the Centre for Addiction and Mental Health in Toronto conducted a trial involving 40 healthy adult volunteers who regularly use cannabis. Participants consumed soft-chew gummies with varying doses of THC (0, 2, 10, or 20 milligrams) during four separate sessions. They were tested using a driving simulator at two, five, and 24 hours after consumption. The study found that those who consumed 10- and 20-milligram gummies exhibited impaired driving skills, including lane control issues and slower reaction times, while their blood THC levels remained below the standard legal cutoff of 5 nanograms per milliliter (ng/mL).
The highest blood THC concentrations recorded were 1.6 ng/mL after the 10-milligram dose and 3.4 ng/mL after the 20-milligram dose. In contrast, participants in a previous study involving smoked cannabis showed similar impairment levels with an average peak blood THC concentration of 30.3 ng/mL.
The study noted limitations, including the use of virtual simulators instead of real roads, which eliminated physical risk, and the focus on regular cannabis users, leaving the effects on occasional users unclear. Minimal residual effects were observed after 24 hours, with subjective feelings of intoxication fading despite slightly higher maximum speeds among participants who consumed the higher doses.
Dr. José Ignacio Nazif-Munoz from the Université de Sherbrooke commented on the implications for law enforcement, stating that the delayed absorption and prolonged effects of edible cannabis complicate the interpretation of biological measures, indicating a need for more reliable roadside testing tools.