Drivers who consume cannabis edibles may experience significant problems with road performance and reaction times even when blood tests show THC concentrations well below a commonly used legal threshold, according to a new study.

The findings, published in JAMA Network Open, point to a potential enforcement challenge in Canada and some U.S. jurisdictions. Standard blood testing may not reliably reflect the level of driving impairment associated with edible cannabis.

Earlier research examining cannabis and driving has largely involved smoked or inhaled marijuana. Those forms more rapidly increase bloodstream levels of THC, or tetrahydrocannabinol, the main psychoactive compound in cannabis.

Edibles are absorbed more slowly, however, and can impair driving while blood THC concentrations remain relatively low. Researchers found that higher edible doses were associated with difficulty controlling lane position, slower reactions and greater variations in speed.

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Researchers at the Centre for Addiction and Mental Health in Toronto conducted a trial involving 40 healthy adult volunteers who regularly use cannabis. Each participant attended four separate sessions and consumed gummies containing either 0, 2, 10 or 20 milligrams of THC.

The participants completed driving simulator tests at two, five and 24 hours after consuming the gummies. Researchers monitored how well they remained in their lanes, how quickly they responded to events on the road and how consistently they controlled their speed.

The research team also collected blood samples and asked participants to rate their level of intoxication. Those measurements allowed investigators to compare driving performance, blood THC concentrations and the participants’ own assessments of how they felt.

Driving impairment appeared at two and five hours after participants consumed the 10 and 20 milligram gummies. Compared with placebo, drivers who received those doses showed greater variability in their lane position.

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The 20 milligram dose was also associated with slower reaction times and less consistent speed control. At the same time, participants reported less willingness to drive and reduced confidence in their driving ability after consuming the gummies.

The highest blood THC concentrations averaged 1.6 nanograms per milliliter following the 10 milligram dose. After the 20 milligram dose, the highest concentrations averaged 3.4 nanograms per milliliter.

Both averages remained below the commonly used 5 ng/mL threshold, although legal THC limits differ by jurisdiction. Some individual blood samples exceeded 5 ng/mL, according to the study findings.

The research team compared those results with its previous study of smoked cannabis. Participants in that earlier research showed similar impairment while reaching an average peak blood THC concentration of 30.3 nanograms per milliliter, nearly 10 times higher than the average for the 20 milligram edible group.

The edible cannabis study had several limitations. Testing took place in virtual driving simulators rather than on actual roads, so participants did not face real physical danger during the experiments.

The study also included regular cannabis users, leaving it unclear how occasional users might respond to comparable doses. Therefore, the results do not establish how the same amounts would affect people who use cannabis less frequently.

Researchers detected minimal residual effects after 24 hours. Maximum speed remained slightly higher among participants who received the 10 and 20 milligram doses, but their subjective feelings of intoxication had completely disappeared.

In an accompanying commentary, Dr. José Ignacio Nazif Munoz of the Université de Sherbrooke said the biological differences create difficulties for law enforcement. Slowly absorbed edible cannabis may leave blood tests unable to identify some drivers whose performance is unsafe.

"The delayed absorption and prolonged duration of effects of edible cannabis distinguish it from smoked or inhaled cannabis, complicating the interpretation of biological measures," he wrote. The findings leave policymakers facing the challenge of finding more reliable tools for roadside enforcement.