Driving after consuming cannabis edibles can lead to significant impairments in road performance and reaction times, even when blood tests for THC remain below commonly used legal thresholds, according to a new study.
The findings, published in JAMA Network Open, highlight a potential challenge for impaired-driving enforcement, particularly in jurisdictions where cannabis is legal.
Unlike smoked marijuana, which rapidly increases THC (tetrahydrocannabinol) levels in the bloodstream, edibles are absorbed more slowly. This delayed absorption can result in driving impairment even with relatively low blood THC concentrations.
Researchers at the Centre for Addiction and Mental Health in Toronto conducted a trial involving 40 regular cannabis users. Participants consumed gummies containing 0, 10, or 20 milligrams of THC and were then tested using a driving simulator at two, five, and 24 hours after consumption. The tests measured lane control, reaction times, and speed consistency.
Results indicated that participants who consumed 10-mg and 20-mg THC gummies experienced significant driving impairment at the two- and five-hour marks. Those in the 20-mg group showed slower reaction times and less consistent speed control, along with greater variability in lane position compared to a placebo group. Participants also reported reduced confidence in their driving abilities and a lower willingness to drive after consuming the higher doses.
During the study, the highest average blood THC concentrations for the 10-mg and 20-mg edible doses were 1.6 and 3.4 nanograms per milliliter, respectively. These levels were below the common legal cutoff of 5 ng/mL used in many roadside traffic stops, although THC limits can vary by jurisdiction, and some individual samples in the study exceeded this threshold.
For comparison, a previous study involving smoked cannabis showed similar levels of impairment in participants who reached an average peak blood THC concentration of 30.3 ng/mL, nearly ten times higher than the 20-mg edible group. This suggests edibles can cause impairment at much lower detectable blood THC levels.
The study did note limitations, including the use of driving simulators instead of real-world driving and a focus on regular cannabis users, meaning the effects on occasional users remain unclear. Residual effects were minimal after 24 hours, with subjective feelings of intoxication having faded, although maximum speed remained slightly higher in the 10-mg and 20-mg dose groups.
In an accompanying commentary, Dr. José Ignacio Nazif-Munoz of the Université de Sherbrooke pointed out that these biological differences complicate law enforcement's ability to identify impaired drivers. The slow absorption and prolonged effects of edibles mean that standard blood tests may not accurately reflect a driver's impairment at the time of a roadside stop, prompting a need for more reliable detection methods.
