Driving after consuming cannabis edibles can lead to significant impairment in road performance and reaction times, even when blood tests for THC (tetrahydrocannabinol) remain below legal limits, according to a study published in JAMA Network Open. This finding presents a potential challenge for impaired-driving enforcement in jurisdictions where cannabis is legal.
Previous studies on cannabis and driving have primarily focused on smoked or inhaled marijuana, which causes rapid spikes in THC levels. Edibles, however, are absorbed more slowly, meaning they can impair driving abilities even with lower concentrations of THC in the bloodstream. Researchers found that drivers who consumed higher doses of edible THC struggled with maintaining their lane position, exhibited slower reaction times, and showed greater fluctuations in speed. Crucially, their blood THC levels remained below the common roadside legal cutoff used to determine impairment.
To investigate the effects of oral cannabis on driving, researchers at the Centre for Addiction and Mental Health in Toronto conducted a trial involving 40 healthy adult volunteers who regularly use cannabis. Participants attended four sessions, consuming soft-chew gummies containing 0, 2, 10, or 20 milligrams of THC. Their driving performance was assessed using a simulator at two, five, and 24 hours after consumption. The assessments tracked lane control, reaction times to road events, and speed consistency. Blood samples and self-reported intoxication levels were also collected.
The results indicated driving impairment at the two- and five-hour marks after consuming 10- and 20-milligram gummies. Compared to a placebo, participants in these groups showed greater variability in their lane position. The 20-milligram dose was also associated with slower reaction times and less consistent speed control. Participants also reported a decreased willingness to drive and lower confidence in their driving abilities after consuming the gummies.
The highest average blood THC concentrations observed were 1.6 nanograms per milliliter after the 10-milligram dose and 3.4 nanograms per milliliter after the 20-milligram dose. These levels were below the frequently used 5-ng/mL threshold, though THC limits can vary by jurisdiction, and some individual samples exceeded this limit.
In contrast, a previous study on smoked cannabis by the same research team showed similar levels of impairment occurring with average peak blood THC concentrations of 30.3 nanograms per milliliter, nearly ten times higher than the levels seen in the 20-milligram edible group. This suggests that impairment from edibles may not correlate with blood THC levels in the same way as smoked cannabis.
The study acknowledged certain limitations, including the use of driving simulators instead of real-world driving conditions, meaning participants faced no actual physical risk. The research also focused on regular cannabis users, so the effects on occasional users remain unclear. Additionally, residual effects were minimal after 24 hours, with subjective feelings of intoxication having faded, although maximum speed remained slightly higher in the 10- and 20-milligram groups.
An accompanying commentary highlighted that the delayed absorption and prolonged effects of edible cannabis complicate the interpretation of biological measures for law enforcement. The findings suggest that standard blood tests might not reliably identify drivers impaired by edibles, prompting a need for more effective roadside detection tools.