The study, published in the journal Psychopharmacology, and led by PhD student Thomas Arkell, compared the effects of standard THC-dominant cannabis, cannabis containing equivalent amounts of THC and CBD, and placebo cannabis on simulated driving and cognitive performance.
In a randomised, double-blind, crossover design, 14 healthy volunteers with a history of light cannabis use attended three outpatient experimental test sessions in which simulated driving and cognitive performance were assessed.
The THC-dominant cannabis was vaporised at a dose (125 mg) that caused strong feelings of intoxication in users and a reluctance to drive, the study found.
When tested on a sophisticated driving simulator, those given THC were impaired for up to four hours on a demanding car following task, although not on a more straightforward standard highway driving task. The study found the type of impairment seen with THC-dominant and THC/CBD equivalent cannabis involved greater lane-weaving.
However, on other measures intoxicated participants were somewhat safer, tending to leave a larger gap between them and the car in front and showing no tendency to speed.
CBD and driving impairment
Contrary to prediction, the study found that the addition of CBD to THC does not reduce feelings of intoxication, nor did it lessen driving impairment compared with standard THC-dominant cannabis. In some circumstances, the study found that the presence of CBD even exacerbated THC-induced impairment.
The THC-containing cannabis had only a modest impairing effect on simulated driving performance. The only performance measure to significantly worsen with cannabis was lane-weaving and participants in both the THC-dominant and balanced THC/CBD groups tended to leave a larger gap between them and the car in front compared with the placebo group.
This study is the first in a series of cannabis and driving-related research planned by the Lambert Initiative. A study assessing the accuracy and sensitivity of roadside drug testing procedures will be published in the near future; an expanded version of this study involving real on-road driving is now underway in collaboration with the University of Maastricht (Netherlands); as well as an upcoming trial assessing simulated driving and cognitive performance using CBD-only cannabis products.
“It is imperative to better understand the effects of cannabis on driving so legal frameworks can be updated and unambiguous advice can be given to patients, all grounded in high quality science,” said Professor McGregor.