Edible cannabis can impair driving while blood tests stay below legal limits

In a controlled trial, 40 adult volunteers who regularly consume cannabis attended four sessions in which they ate soft-chew gummies with varying THC amounts (0, 2, 10, 20 mg). Using a high-fidelity driving simulator, researchers measured lane position, reaction time and speed consistency at two, five and 24 hours post-consumption. At the 10-mg and 20-mg levels, drivers showed greater lane deviation, slower response to road events and more speed fluctuation, while their blood THC averaged 1.6 ng/mL and 3.4 ng/mL respectively—both below the typical 5 ng/mL legal limit.

Participants also indicated reduced willingness to drive and lower confidence after the higher doses. The study, published in JAMA Network Open, notes that edibles’ delayed absorption can mask impairment in standard roadside tests, posing a challenge for law-enforcement agencies. Limitations include the use of a simulator rather than real roads and a focus on regular users, leaving effects on occasional users uncertain.