Vaping tetrahydrocannabinol (THC) impairs driving performance, while the effects of cannabidiol (CBD) are uncertain, according to a crossover trial in The Journal of the American Medical Association (JAMA). THC-dominant cannabis is typically used recreationally, while CBD-dominant products may be prescribed for medicinal use.
Twenty-six healthy adults who used cannabis occasionally participated in four separate experimental sessions in randomised order, at least 1 week apart: They inhaled vaporised THC-dominant cannabis, THC/CBD-equivalent cannabis, CBD-dominant cannabis, or placebo, and then underwent hour-long, on-road driving tests at 40 minutes and again at 240 minutes after vaping.
Participants showed impaired driving — measured using a marker of lane weaving and swerving — during the first test after vaping THC and THC/CBD, relative to placebo. The impairment was comparable to that observed at a blood alcohol concentration of 0.05%. There was no significant impairment during the later test.
Meanwhile, CBD did not appear to affect driving. The authors note, however, that "the effect size ... may not have excluded clinically important impairment, and the doses tested may not represent common usage."
While the findings from this trial do not support the conclusion that it is safe to drive after consuming CBD, it is clear that THC did impair driving.