Cannabis effects on driving are not nearly as predictable as those of alcohol, said Professor Iain McGregor, academic director of the Lambert Initiative for Cannabinoid Therapeutics.
Most street cannabis contains large quantities of THC, the chemical in cannabis that gets people ‘high’, but there is increasing use of medicinal products that also contain cannabidiol (CBD), a non-intoxicating cannabinoid best known in the treatment of severe epilepsy but also useful in treating anxiety, psychosis and pain. It has often been proposed that administering CBD may reduce some of the impairment caused by THC.
“This is a red hot issue for patients using medicinal cannabis many of whom are being told by their doctors not to drive under any circumstances,” said Professor McGregor.
But on some measures, cannabis does not impair driving to the same extent as alcohol, and prescription drugs such as benzodiazepines and opioids.
“And while it is illegal in NSW to drive with any amount of THC in your system, other countries and jurisdictions have more refined laws that attempt to more clearly link driving prohibition to THC-induced impairment.”
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.
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.https://www.sydney.edu.au/news-opinion/ ... iving.html