Epidemiological studies have shown that cannabis is associated with an increased risk of car crashes.
Whether or not, cannabidiol, the derivative widely used as medicinal cannabis, also impairs driving performance is largely unknown.
Cannabis is available in three different varieties (or technically, chemovars) which are cannabidiol (CBD) dominant, tetrahydrocannabinol (THC) dominant and a CBD-THC equivalent mixture. Individuals inhale the THC dominant form because of its intoxicating effects. Whether the CBD dominant form, used medicinally, has sedating properties is currently equivocal.
As a result, researchers from the Lambert Initiative for Cannabinoid Research, University of Sidney, Australia, conducted a randomised trial to examine the extent and duration to which each of the different cannabis chemovars impaired driving performance. They recruited 26 individuals with a history of occasional cannabis use and employed a cross-over design study that involved four on-road driving periods, one week apart. For the study, the participants inhaled one of vapourised cannabis containing 13.75mg CBD, THC, CBD-THC equal mixture or placebo. The driving tests twice; 40 to 100 minutes and 240 to 300 minutes post-vaporisation. The primary outcome was the mean standard deviation of lateral position (SDLP), which measures the extent of lane weaving, swerving and overcorrecting.
The mean age of the 26 individuals was 23.2 years (including 16 women), all of whom completed the 8 driving tests. At 40 to 100 minutes and following inhalation of cannabis, the mean SDLP after CBD was 84.07cm, 86.94cm after THC, 84.07cm after the CBD-THC mixture and 84.41cm after placebo and these values remained similar between 240 and 300 minutes. Pairwise comparisons revealed that both SDLP values for THC and the THC-CBD were higher than placebo at 40 to 100 minutes, but not between 240 and 300 minutes. In other words, both THC and the combination but not, CBD, impaired driving performance in the short-term, but that this effect was abolished in the longer term.
Although a limitation in the study, recognised by the authors, was that the dosages used might not reflect common usage, their data implied that CBD did not affect driving ability.
Arkell TR et al. Effect of cannabidiol and delta-nine tetrahydrocannabiniol on driving performance. A randomised clinical trial. JAMA Net Open 2020 doi:10.1001/jama.2020.21218