A new study has found that driving after consuming cannabis edibles can significantly impair reaction times and road performance, even when blood THC levels remain well below the legal limit used in roadside enforcement. Published in JAMA Network Open, the research highlights a gap in impaired-driving laws in Canada and parts of the US, since most existing legal thresholds were designed around smoked cannabis, which raises blood THC far more rapidly than edibles do.

Researchers at Toronto's Centre for Addiction and Mental Health tested 40 regular cannabis users who ate gummies containing 0, 2, 10 or 20 milligrams of THC, then assessed their driving on a simulator at two, five and 24 hours later. Those who took the 10mg and 20mg doses showed poorer lane control, slower reactions and inconsistent speed at two and five hours, despite average peak blood THC readings of just 1.6 and 3.4 nanograms per millilitre—far below the common 5 ng/mL legal threshold and roughly a tenth of the levels seen after smoking cannabis in a comparable earlier study. A commentator noted this discrepancy poses a real challenge for law enforcement, as standard blood tests may fail to detect unsafe drivers who have consumed edibles.