Marijuana users may face hidden driving danger that standard tests can miss
A new study warns that driving after eating cannabis edibles can significantly impair road performance and reaction times, even if blood tests show THC levels far below the legal limit.
The findings, published in JAMA Network Open, highlight a potential challenge for impaired-driving enforcement in Canada and some U.S. jurisdictions.
Most previous cannabis-driving studies have involved smoked or inhaled marijuana, which more rapidly spikes levels of THC (tetrahydrocannabinol, the main psychoactive compound in cannabis) in the bloodstream.
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Edibles are absorbed more slowly and can produce driving impairment even when blood THC concentrations remain relatively low.
Researchers found that drivers who consumed higher doses of edible THC struggled with lane control, had slower reaction times and experienced greater speed fluctuations. Yet, their blood tests stayed below the standard legal cutoff used to prove impairment at roadside traffic stops.
To assess how oral cannabis impacts driving, researchers at the Centre for Addiction and Mental Health in Toronto conducted a trial with 40 healthy adult volunteers who regularly use cannabis.
Each participant attended four separate sessions in which they consumed soft-chew gummies containing either 0, 2, 10 or 20 milligrams of THC.
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The researchers then tested the drivers using a driving simulator at two, five and 24 hours after consumption, tracking how well drivers stayed in their lane, how quickly they reacted to road events and how consistently they maintained their speed.
Blood samples and self-reported ratings of intoxication were also collected.
The results showed driving impairment at two and five hours after consuming 10- and 20-milligram gummies. Compared with placebo, participants who consumed those doses had greater variability in lane position, while the 20-milligram dose was also linked to slower reaction times and less consistent speed control.
Participants also recognized their own limitations, reporting a lower willingness to drive and reduced confidence in their driving skills after taking the gummies.
The highest blood THC concentrations averaged 1.6 nanograms per milliliter after the 10-milligram dose and 3.4 nanograms per milliliter after the 20-milligram dose. Those levels were below the commonly used 5-ng/mL threshold, although THC limits vary by jurisdiction and some individual samples exceeded 5 ng/mL.
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By comparison, participants in the team’s previous smoked-cannabis study showed similar levels of impairment while reaching an average peak blood THC concentration of 30.3 nanograms per milliliter, nearly 10 times higher than the 20-milligram edible group.
The study did have some limitations. Tests were conducted on virtual simulators rather than actual roads, meaning drivers faced no real physical risk during the experiments. It also focused on regular cannabis users, so it is unclear how occasional users might react to similar doses.
Additionally, minimal residual effects were detected after 24 hours. While maximum speed remained slightly higher among participants on the 10-milligram and 20-milligram doses, subjective feelings of intoxication had completely faded.
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In an accompanying commentary, Dr. José Ignacio Nazif-Munoz of the Université de Sherbrooke noted that these biological differences pose a challenge for law enforcement.
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Because edibles are absorbed slowly through the digestive system, standard blood tests may fail to identify unsafe drivers, leaving policymakers searching for more reliable roadside tools.
"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.