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Cannabis is commonly used in Canada. According to a recent survey, 17.% of Canadians reported using cannabis at least once in the past three months. Around 6% said they used cannabis "daily or almost daily."
Driving after using cannabis is relatively common in Canada. Fifteen percent of cannabis users with a driver's license reported driving within two hours of use at least once in the last year. (It's safest to wait at least six hours before driving.) Two recent studies in BC revealed that around 8 of drivers who sustained injuries in car crashed tested positive for cannabis, among other substances.
Cannabis contains THC (the short name for the mind-altering chemical in cannabis). THC can impair our ability to drive. When THC is in our blood, it may affect our tracking ability, reaction time, sight, concentration and memory. THC can also compromise our ability to handle unexpected events, such as a child stepping out onto the street.
(Note: You may have heard about CBD, the short name for a therapeutic compound found in cannabis. CBD is not mind-altering and does not affect driving.)
The way THC affects us depends on many factors, including the strain of cannabis and our experience with the substance. Evidence suggests regular users of THC may be more tolerant of its impairment effects. But this doesn't mean it's OK to drive if you're a regular cannabis user. Cannabis can impair many aspects of functioning that affect safe driving, even in regular users.
Evidence shows cannabis use increases our risk of being in a vehicle crash. What's more, using cannabis in combination with alcohol puts us at significantly higher risk of harm. THC can magnify the effects of alcohol. In 2014, nearly one in five fatally injured drivers tested positive for THC, among other substances.
For some people, the benefits of cannabis seem to outweigh the risk, including risks related to driving. For example, there are people with medical issues who may be using cannabis (THC and CBD) throughout the day to function and participate in life as a "regular person." Others may be using it to cope with the stress, routine, or boredom of their job or occupation.
"I'm a very anxious type of person. I used cannabis just to be able to relax while working. It kept me awake and, believe it or not, helped me focus better. I knew there were risks, like losing my Class 1 licence. But the benefits outweighed the risks 100%"—retired truck driver and heavy equipment operator
Other reasons people risk driving after using cannabis may involve their beliefs about drug use. For example, some people think cannabis is not very intoxicating and therefore not much of an obstacle to driving. Yet cannabis can be a depressant, which means it slows down activity in our central nervous system. This can equate to slower brain function, poor concentration and confusion.
Some cannabis users who drive may understand the impairment effects of cannabis and avoid getting behind the wheel within the first hour after using it. However, other users—younger male drivers in particular—may be at increased risk of what most would consider to be "reckless driving." It is always safest to wait six hours before driving after using cannabis.
The police use a range of tools to come to a conclusion about a person's ability to drive. The smell of the drug on the driver or in the vehicle, red eyes and dilated pupils, and lapses of attention and concentration may suggest impairment. These clues give the police permission to ask drivers to perform three road-side tests: horizontal gaze nystagmus (involuntary jerking of the eyes when moved to the side), a one-leg stand, and a walk and turn. Drivers may also be asked to provide a sample of oral fluid. Some officers receive special training on detecting cannabis impairment and may issue a roadside suspension based on their judgement alone.
Yet, cannabis impairment testing is inherently tricky. THC can still register in a person's body a long time after they've used the drug. This means a driver can test positive for THC even when well below intoxication or impairment level. Furthermore, it is not clear what blood THC levels actually indicate impairment and this seems to vary depending on the mode of ingestion.
If a driver is deemed impaired, they may be subject to fines, license suspensions, and other penalties, including increased insurance costs. See RCMP site for more details.
There are personal and social costs too. Losing your license can affect your self-esteem and confidence, your reputation among family and friends, and your job (if your job involves driving or you need to drive to get to work). And an impaired driving charge can stay on your driving record for a long time.
Think of your well-being and that of others. What message are you sending to others if you are willing to take risks such as driving when impaired or riding with someone who may be impaired?
Check your beliefs. How do they match up with those of other people you know? What are they based on? Being honest about your own experience and giving real consideration to other opinions can help you make good decisions. Doing a little research, from credible sources, may also help you become better informed about the properties and effects of cannabis.
Make changes, if you want to. It can be refreshing to reflect on our behaviour, including drug use, and think about small things we might want to do to increase our health and well-being. For example, we could try taking a walk after work before or instead of unwinding with cannabis or other substances.
Some people will choose to use cannabis, regardless of rules or regulations. For those considering using cannabis, here are some things to think about and ways to reduce harm.
Before you use cannabis, ask yourself...
Do I really want to use it? Sometimes cannabis helps. Sometimes it makes things worse.
Can I trust my source? Legal cannabis sources are tested for quality while street cannabis is not.
How much THC is in it? THC or delta-9-tetrahydrocannabinol is the most well-known cannabinoid that causes impairment. Too much THC can also cause other unwanted effects (e.g., psychosis, paranoia).
How much CBD is in it? Cannabidiol or CBD is another cannabinoid. Unlike THC, CBD does not cause impairment. There is some evidence that CBD may block or lower some of the effects of THC and may contribute to the health benefits associated with cannabis use.
It's safer to...
Avoid using too much too often, especially if you're young. Human brains are not fully developed until early adulthood. Regular use (daily or almost daily) over time can lead to dependence. You may start needing it just to feel normal.
Wait at least six hours before driving or operating machinery.
Avoid smoking. Vaping or edibles are better options because they are not as harmful to your lungs. If you do smoke, don't hold in the smoke. 95% of the THC is absorbed immediately.
Go slowly when eating or drinking cannabis. You can get higher than expected. Try a little and wait an hour before using more. Same advice when trying a new type of cannabis—go slowly.
Avoid mixing substances. Adding tobacco to a joint means adding another drug along with cancer-causing toxins. Drinking alcohol while using cannabis intensifies the effects, including impairment, and makes them last longer than expected.
Skip cannabis if you (or a member of your family) have a history of psychosis or a substance use disorder. Cannabis use increases risk that symptoms of these conditions will reappear or get worse. If you are pregnant or breastfeeding, it’s safest to avoid using cannabis.
Closing the Research Gap on Cannabis and Mental Health. Mental Health Commission of Canada, 2019
Clearing the Smoke on Cannabis: Cannabis Use and Driving—An Update. Canadian Centre on Substance Use and Addiction, 2019
Cannabis Use and Driving: Evidence Review. Canadian Drug Policy Coalition (CDPC) Simon Fraser University, 2017
Drug-Impaired Driving. Government of Canada, 2020
Impaired Driving Investigations. Royal Canadian Mounted Police, 2018
About the author
The Centre for Addictions Research of BC is a member of the BC Partners for Mental Health and Substance Use Information. CARBC is an official research centre of the University of Victoria. The Centre collaborates with service providers and other stakeholders to support effective public education, system planning, and service delivery. For more, visit www.carbc.ca.