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Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial.

Thomas R Arkell ^1 2 3 4, Frederick Vinckenbosch ⁴, Richard C Kevin ^1 2 5, Eef L Theunissen ⁴, Iain S McGregor ^1 2 5, Johannes G Ramaekers ⁴

PMID: 33258890

An original investigation conducted by Arkell et al. assayed the effects of cannabidiol (CBD) compared to ∆⁹-Tetrahydrocannabidiol (THC) on driving performance. The study investigated the effects of vaporized THC-dominant versus CBD-dominant cannabis on driving performance; participants were between 20-50 years of age, had ample driving experience, reported infrequent cannabis use, but had at least 10 prior experiences with cannabis.

Participants received vaporized cannabis containing either 13.75 mg THC, a combination of 13.75mg THC & 13.75mg CBD (THC/CBD group), just 13.75mg CBD, or a placebo.

Participants took a driving test on a 100-km highway circuit while maintaining a speed of 59 miles per hour and a steady lateral position (SDLP)–the distance between the vehicle and the lane boundary to the left of the vehicle, measured using an external apparatus. Cars were equipped with brakes for a licensed driving instructor who monitored safety.

The THC condition had a significantly increased SDLP compared to both the CBD condition and placebo (P < .001). There was no significant difference in SDLP between the CBD condition and placebo, indicating that CBD alone didn’t significantly alter driving performance. But 300 minutes after THC administration, there was no longer a significant difference in SDLP compared with placebo (P > .99) indicating that the effects had worn off.

A secondary outcome, feeling ‘confident to drive’ was significantly decreased in the THC and THC/CBD condition (P < .001) compared to placebo, demonstrating observably altered subjective effects in the THC group. There was also a main effect of condition on performance on a cognitive task, the Digit Symbol Attention Task that measures processing speed and working memory. Overall, the THC group showed the most significant difference in performance, while the CBD-dominant cannabis failed to produce significant cognitive or psychomotor impairment, compared with placebo.