How does cannabis affect driving?
When you drive, your hands, eyes and feet control the vehicle, and your brain controls your hands, eyes and feet. To drive safely, you need to be alert, aware and able to make quick decisions while navigating the road and other vehicles.
Cannabis can increase the risk of a collision by affecting the normal functions of the brain and body. It makes it harder to stay in your lane. It also slows reaction times, so it takes longer to brake or respond to sudden events on the road. Drivers who feel high might try to compensate for this by slowing down and keeping more distance from the car ahead but these adjustments aren't enough to make driving safe.
What is THC?
THC (also known as delta-9-tetrahydrocannabinol) is a chemical in cannabis that's responsible for its intoxicating and impairing effects. The more cannabis someone uses, the more THC enters their bloodstream, and the higher they feel. THC can be detected by testing blood or saliva and, if found, indicates recent cannabis use. Related chemicals, known as “breakdown products,” can be detected in urine for many days afterward.
How long do the effects of cannabis last?
This depends on several factors, including age, sex, dose, frequency of use and the type of cannabis consumed. In general, edibles last longer than smoked or vaped cannabis, and higher doses last longer than lower. Most current research on this is based on low-to-moderate doses of inhaled cannabis. It suggests that effects can last up to six hours, and sometimes up to 12. Effects likely last even longer with high-potency products now on the legal market, and with new oral products like sprays and sublingual strips.
What about newer forms of cannabis?
Since legalization in 2018, the number of cannabis products has exploded. Newer types such as edibles and vapes are increasingly popular, and novel products like beverages and concentrates (highly potent waxes, oils, rosins) are also available. These products have emerged so quickly that research has not kept up. No matter what you consume, you should always be cautious and avoid driving after using cannabis.
How common is driving after using cannabis?
According to the Canadian Cannabis Survey (2024), among people who used cannabis in the past year:
- 16 per cent said they drove within two hours of smoking or vaping cannabis
- 10 per cent said they drove within four hours of ingesting cannabis
When people in Canada were asked whether cannabis impairs driving, 86 per cent said yes. Among cannabis users, that number was 79 per cent.
Has driving under the influence of cannabis changed since legalization?
More people are using cannabis since it was legalized in Canada. This has increased the risk of more people driving under the influence of cannabis.
What does the law say about driving under the influence of cannabis?
In Canada, if police suspect you've driven after using cannabis, they will give you a blood test. The legal consequences depend on the THC level found in your blood. If they find:
- 2 ng/mL to 5 ng/mL, it is a summary conviction (less serious) offence
- 5 ng/mL or more, it is a hybrid (more serious) offence
Police can also test saliva at the roadside. If THC levels in your saliva are above 25 ng/mL, they may consider you impaired. In most cases, police will use a combination of saliva testing and a Standardized Field Sobriety Test (checking your balance, coordination and motor skills) to guide their decision to take you to a station for a blood test.
Using cannabis can affect your ability to drive safely. If you use cannabis in any form, understand the risks and plan ahead.
Developed in collaboration with the Driving Simulator Lab in the Institute for Mental Health Policy Research.
