Higher cannabis potency increases driving risk, study finds

Participant using a multi-screen driving simulator at the CAMH Driving Simulation Laboratory.

Driving while impaired by cannabis is known to increase the risk of motor vehicle collisions; yet, in 2024, about 16 per cent of Canadians who used cannabis drove within two hours of smoking or vaping, according to Health Canada’s Canadian Cannabis Survey.

A new study led by researchers at the Centre for Addiction and Mental Health (CAMH), including Leslie Dan Faculty of Pharmacy professor and affiliated scientist Beth Sproule, examined how the potency of smoked cannabis can impact driving ability, providing evidence to support lower-risk cannabis use.

“From a public health lens, some of the most common cannabis-related harms are related to injuries from driving collisions,” says Sproule, also a scientist with the Institute for Mental Health Policy Research at CAMH. “The increasing use of cannabis in recent years, paired with the higher potency of THC in many cannabis products, means it is really important to understand how cannabis use affects driving.”

“The increasing use of cannabis in recent years, paired with the higher potency of THC in many cannabis products, means it is really important to understand how cannabis use affects driving.”

THC (tetrahydrocannabinol) is the psychoactive component of cannabis that results in impairment, and Sproule explains that potency of THC in cannabis has increased in recent years. A recent study reported that cannabis had an average potency of about 4 per cent THC in 1995 and 14 per cent in 2019, while the potency of some cannabis products today is close to 30 per cent. In some cases, users may not realize the potency of the products they are using and the effect that the potency can have.

In this new study, published in Scientific Reports, a research team including experts in addiction science, pharmacology and roadway safety examined how different potencies of smoked cannabis affected driving performance. As a pharmacist, Sproule’s role on the team was related to advising on the pharmacokinetics of cannabis.

CAMH has a Driving Simulation Laboratory, which includes a simulator designed to closely replicate driving in the real world. By measuring participants’ performance with the simulator, researchers can examine the effects of mental health or different substances on driving ability and generate scientific evidence that can inform road safety policies.

“Driving is a complex behaviour that requires the coordinated effort of many cognitive and psychomotor skills. Understanding drug-impaired driving relies on a range of methods to identify which specific cognitive or psychomotor skills are affected, and how this affects driver safety,” says Justin Matheson, research manager at the Driving Simulation Laboratory. “The advantage of using a driving simulator is that it provides a real assessment of driver behaviour and performance in a controlled environment. This approach is safe for the research participant and allows us to isolate the specific aspects of driving that are affected.”

Study raises awareness of effects of increased potencies

The driving simulator was key to this study, as participants’ driving performance was measured before smoking cannabis and at 30 and 90 minutes after smoking a placebo or low-, medium- or high-potency cannabis. The researchers also measured participants’ blood THC concentrations.

The researchers found that after smoking medium- or high-potency cannabis, participants using the driving simulator had higher maximum speeds, more variability in speed, more variability in lane position (“weaving”), and slower reaction times compared to those who smoked low-potency products or placebos. Blood THC concentrations showed a similar effect, with higher THC concentrations resulting in generally poorer driving performance.

Importantly, the effect of potency was similar at 30 to 90 minutes, suggesting that this is not long enough to wait to reduce impairment. Canada’s Lower-Risk Cannabis Use Guidelines currently recommend waiting at least six hours after using cannabis before driving.

“These results aren’t particularly surprising, but it’s important to measure and document these subjective effects, which has been a challenge,” says Sproule. “The current guidelines already include warnings about driving and cannabis use, and this research provides more evidence to support those guidelines, especially by paying attention to the strength of the cannabis you’re smoking and waiting before driving.”

Importantly, this study was specifically related to smoked cannabis, but the research team is working on another study to examine how edible cannabis – an increasingly common method of consumption – affects driving performance, which may be different because of the different route it takes to reach the brain. They are also doing a follow-up study to examine whether cannabis potency affects driving performance of men and women differently.

“By understanding the impact these changes have on road safety, our research can help to inform drivers who use cannabis to make responsible decisions.”

“Since cannabis legalization in Canada, we’ve seen dramatic changes in the types of products available to consumers, and these changes are reflected in patterns of use with a much greater diversity of products and higher doses of THC,” says Matheson. “By understanding the impact these changes have on road safety, our research can help to inform drivers who use cannabis to make responsible decisions.”

Sproule agrees that demonstrating the relationship between increasing THC potencies and driving performance is important for informed decision-making.

“It helps to raise awareness and remind people that even if you’re using legal or medical cannabis, some of these products can have very high THC concentrations. It’s important to be aware of what you’re taking and what the effects and impairments can be.”