REVIEW OF: Marcotte TD et al, JAMA Psychiatry 2022;79(3):201–209

STUDY TYPE: Double-blind, placebo-controlled parallel randomized clinical trial

Everyone knows that driving while under the influence of alcohol is dangerous. But what about cannabis? The effect of cannabis on driving ability is not well understood but is becoming increasingly relevant as cannabis laws are liberalized around the nation. Researchers designed a double-blind placebo-controlled trial to determine how THC affects driving ability and how long its effects last.

The study recruited 191 participants who had used cannabis ≥4 times in the past month. Sixty-two percent were male, 43% were non-Hispanic White, and ages ranged from 21 to 55. Participants abstained from cannabis for at least 48 hours and then were randomly assigned to one of three groups: low THC, high THC, or placebo.

On testing day, participants were given cannabis cigarettes and instructed to smoke “as you would at home.” The THC content of the cannabis was 13.4% in the high THC group (comparable to typical whole-plant cannabis purchased at dispensaries today), 5.9% in the low THC group, and 0.02% in the placebo group. Participants completed a driving simulation 30 minutes, 90 minutes, 3.5 hours, and 4.5 hours after smoking. The simulation lasted 25 minutes, emulated urban and city driving, and had challenges such as highway merging, driving while attending to an iPad mounted on the dashboard, and maintaining constant distance from a car being driven at varying speed. Performance was quantified by a Composite Drive Score (CDS), which takes variables like proper speed, time out of lane, and lateral position into account. Serum levels of THC were measured at the time of each driving simulation.

Not surprisingly, drivers in the placebo group had significantly better CDS scores than those in the THC groups. THC had a medium effect size on driving performance at 30 and 90 minutes, but performance was equivalent between groups at the 3.5- and 4.5-hour marks. The low THC and high THC groups had similar impairments in driving performance. Importantly, participants were asked about their subjective perception of their readiness to drive. At 30 minutes, 47.5% in the THC groups said they would drive at their current level of intoxication, and this increased to 68.6% at 1.5 hours.

There were a few limitations to this study. Most notably, 92% of participants correctly guessed whether they received THC. The study recruited healthy participants who regularly used cannabis, so the results may not be generalizable to inexperienced THC users or vulnerable populations (eg, the elderly or those with traumatic brain injury). THC effects could very well be more pronounced or longer lived in these individuals. Authors noted that CDS scores in this study are not directly comparable to impairments in driving for those under the influence of alcohol. For a rough point of comparison, the overall effect size is similar to what would be expected for a blood alcohol level between 0.05% and 0.08%.

Carlat Take

Cannabis consumption impairs driving, even if the THC content is on the low side, and subjective perception is not a good predictor of actual driving ability. Tell your patients who use cannabis to abstain from driving for at least three and a half hours after using, even if they feel safe to drive.