THC (Δ9-tetrahydrocannabinol) dominant and THC/CBD (cannabidiol) equivalent cannabis impairs driving performance 40 to 100 minutes after use according to a study published in the Journal of American Medical Association. After 4 hours, affects appear to dissipate.
The researchers conducted a double-blind crossover clinical trial to assess how various concentrations of CBD and THC affect driving performance.
The crossover trial included 26 healthy participants between age 20 and 50 who self-reported cannabis use less than 2 times per week in the previous 12 months and more than 10 lifetime exposures.
The researchers scheduled driving sessions at least 1 week apart. Participants had to refrain from any other cannabis use for the duration of the study and avoid alcohol for 24 hours before each session.
For the session participants vaporized cannabis containing 13.75 mg of THC, 13.75 mg THC and 13.75 mg CBD, 13.75 mg CBD, or a placebo. Participants took a breathalyzer test and a drug test before each session.
Driving tests took place at 40 to 100 minutes and 240 to 300 minutes after consumption. The primary endpoint was mean standard deviation of lateral position (SDLP), which is a method of measuring lane-weaving.
The study states that from 40 to 100 minutes, the mean lateral position was 86.94 cm (95% CI, 81.50 to 91.48) in the THC group, 85.51 cm (95% CI, 81.81 to 89.21) in the THC/CBD group, 84.07 cm (95% CI, 79.40 to 88.74) in the CBD group, and 84.41 cm (95% CI, 80.01 to 88.82) in the placebo group. From 240 to 300 minutes, the mean lateral position was 85.03 cm (95% CI, 80.88 to 89.17) in the THC group, 84.04 cm (95% CI, 80.64 to 87.54) in the THC/CBD group, 84.25 cm (95% CI, 79.85 to 88.65) in the CBD group, and 83.68 cm (95% CI, 79.45 to 87.91) in the placebo group.
Limitations include the fact that the study was limited to healthy volunteers who were occasional cannabis users. How the study applies to more frequent users is unclear. Also, outcomes may differ with higher doses of CBD and THC.
The researchers found “THC-dominant and THC/CBD-equivalent cannabis produced a short-term impairment during experimental on-road driving, as indexed by a significant increase in SDLP measured 40 to 100 minutes following vaporization. In agreement with previous studies involving smoked cannabis or oral THC (dronabinol), this impairment was modest in magnitude.”
Arkell TR, Vinckenbosch F, Kevin RC, Theunissen EL, McGregor IS, Ramaekers JG. Effect of cannabidiol and Δ9-tetrahydrocannabinol on driving performance: A randomized clinical trial. JAMA. 2020 Dec 1;324(21):2177-2186. doi:10.1001/jama.2020.21218