Driving after cannabis edibles can be a lot riskier than many people assume, and the danger does not always show up in a standard blood test. A new study suggests that even when THC levels stay under common legal limits, drivers can still show weaker lane control, slower reactions, and uneven speed for hours.
The research, published in JAMA Network Open, puts a spotlight on a growing problem for roadside enforcement. Smoked cannabis has been studied more often, but edibles behave differently in the body and can leave officers with a misleading picture if they rely too heavily on blood THC numbers alone.
That difference matters because edibles are absorbed slowly through the digestive system. Instead of the quick spike seen with smoking, the effects can creep in later and hang around longer, which makes it harder to judge impairment on the spot.
Researchers at the Centre for Addiction and Mental Health in Toronto tested 40 healthy adults who regularly use cannabis. Each participant went through four sessions and consumed soft-chew gummies with 0, 2, 10, or 20 milligrams of THC.
The drivers were then put through a simulator at two, five, and 24 hours after taking the gummies. The team watched for classic warning signs of bad driving, including drifting within the lane, slower responses to road events, and sloppy speed control.
The higher doses told a pretty blunt story. People who took the 10- and 20-milligram gummies had more trouble staying centered in their lane, and the 20-milligram dose was tied to slower reactions and more speed swings.
What makes the findings especially tricky is that the blood work did not match the driving behavior. Even with clear impairment, average THC levels stayed below the 5-ng/mL cutoff often used in traffic enforcement, though some individual samples did go above that mark.
The study also showed that drivers had enough self-awareness to know something was off. After taking the gummies, participants were less confident in their driving and less willing to get behind the wheel, which suggests they could feel the problem even when the blood test might not catch it.
That gap between physical impairment and measurable THC levels is where the headache starts for law enforcement. Some jurisdictions use blood limits as a key piece of evidence, but edible cannabis can keep a driver unsafe while the numbers look less dramatic than expected.
The contrast with smoked cannabis is hard to ignore. In the team’s earlier smoked-marijuana research, drivers showed similar impairment while reaching an average peak blood THC level of 30.3 nanograms per milliliter, far higher than the edible group ever reached.
There were limits to the experiment, of course. The testing happened on a simulator rather than a real road, so no one faced actual crash risk, and the volunteers were regular cannabis users, not occasional ones who may react very differently.
Even so, the pattern remained sharp enough to matter. Any lingering effects had mostly faded by 24 hours, but the study still found slight speed increases among those who had taken the 10- and 20-milligram doses, even after the high itself was gone.
An accompanying commentary from Dr. José Ignacio Nazif-Munoz of the Université de Sherbrooke said the biology behind edible cannabis creates a real enforcement puzzle. Because the effects unfold slowly and last longer, roadside decisions based only on lab numbers can miss the people who are still unsafe to drive.
