photo of a car driving through a busy city tunnel

The science of cannabis and driving

7 May 2019
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 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.

Cannabis and driving

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

“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.”

“More research is clearly needed to fully understand how different cannabis products affect driving.”

A person in a driving simulator doing a driving task

A volunteer taking part in a driving and cognitive performance experimental test session.


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.

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