This 2020 randomized clinical trial investigated the effects of CBD-dominant and THC-dominant cannabis on driving performance in occasional cannabis users. The results showed that CBD did not impair driving. However, THC-dominant and THC/CBD-equivalent cannabis produced short-term impairment during experimental on-road driving.
The study involved 26 participants who underwent four driving tests after vaporizing one of the four cannabis formulations: placebo, CBD-dominant, THC-dominant, and THC/CBD-equivalent. The doses of THC and CBD were 13.75mg, and the order of the conditions was randomized and balanced to ensure that each participant had an equal chance of receiving each formulation first.
The results showed that CBD, when administered in a single dose via vaporization, did not impair driving, as measured by the standard deviation of lateral position (SDLP). In contrast, THC-dominant and THC/CBD-equivalent cannabis produced short-term impairment during experimental on-road driving, as indexed by a significant increase in SDLP measured 40 to 100 minutes following vaporization.
At 240 to 300 minutes following consumption, the SDLP did not differ significantly in the CBD, THC, or THC/CBD conditions, relative to placebo. The SDLP ranged from 19.03 cm with CBD-dominant cannabis to 20.59 cm with THC/CBD-equivalent cannabis, indicating that the differences in driving performance between these conditions had dissipated over time.
These findings suggest that the short-term impairment caused by THC-dominant and THC/CBD-equivalent cannabis compared to placebo may not persist beyond several hours after consumption. However, it's worth noting that this study only assessed occasional cannabis users and used moderate THC doses, so the results may not be generalizable to heavier or more frequent users, or to different THC:CBD ratios or doses.
The presence of CBD did not reduce THC impairment of driving, although there were subtle differences in the subjective effects of THC-dominant and THC/CBD-equivalent cannabis despite near-identical THC plasma concentrations. THC/CBD-equivalent cannabis appeared to cause less anxiety, reduced strength of drug effects, and greater confidence to drive than THC-dominant cannabis, particularly at earlier time points.
While the doses of THC used in the study (13.75 mg) were moderate, they caused strong subjective effects including reduced confidence to drive. These findings suggest that co-administration of CBD and THC may provide some benefit in terms of reducing the negative effects of THC, but further research is needed to fully understand this relationship.
These findings show that CBD does not impair driving in occasional cannabis users, while THC and THC/CBD-equivalent cannabis can cause short-term impairment. However, the effect size for CBD-dominant cannabis may not have excluded clinically important impairment, and the doses tested may not represent common usage.
Study Title: Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance
Study Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7709000