The Science of Stoned Driving

Are patients using medicinal cannabis safe to drive?

medicinal cannabis

As medical cannabis becomes more readily available, it is imperative that any risks relating to driving are clarified via thorough research.

photo of a cannabis leaf and a bottle of cannabis oil

New research conducted by the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney shines a light on how the use of different types of cannabis affects driving, feelings of intoxication and cognitive function.

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 is high in 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 also reduce some of the impairment caused by THC.

Cannabis and driving

It is important for people to understand the potential impairment on driving when using cannabis, said Professor McGregor.

“There is no suggestion that driving while stoned is safe and it is important for people to understand potential risks,” said Professor McGregor.

“But it 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.

“And while it is illegal to drive while medically stoned, driving after taking opioids, benzodiazepines and low doses of alcohol is allowed even though they can arguably cause even greater impairment than cannabis. It is clear much more research is needed to fully understand the impacts.”

“Our research on medical cannabis and driving is contributing to the ongoing policy debate regarding issues such as safety, impairment and detection,” said Professor McGregor.

photo of Professor Iain McGregor
Professor Iain McGregor, academic director of the Lambert Initiative for Cannabinoid Therapeutics.

The study, published in the journal Psychopharmacology, compared the effects of standard high-THC cannabis, balanced THC/CBD cannabis, 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 high THC 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 in a complex urban environment, although not on a more straightforward standard highway driving task. The study found the type of impairment seen with high THC 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 did not reduce feelings of intoxication, nor did it lessen driving impairment compared with standard high-THC cannabis. In some circumstances, the study found that CBD exacerbated THC-induced impairment.

Even high-THC 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 high-THC 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.