Smoking Marijuana Impairs Driving for At Least Five Hours

Common advice for people who wish to drive after drinking alcohol is that you should wait one hour for every drink that you had.* For example, if you had 5 drinks, you should wait for 5 hours after your last drink, then you may be safe to drive. The tip is useful for anyone who wishes to drive home after going out to drink. For young adults who use cannabis, the tip might as well be “don’t even think about driving after having a smoke,” since a small dosage can affect your ability to drive 5 hours later.

A joint being smoked. (Chmee2 on Wikimedia Commons)

In a study published this month at CMAJ Open lead by Dr. Tatiana Ogourtsova at McGill University, it was found that participants’ ability to drive was negatively affected even five hours after inhaling the equivalent of one-fifth of a usual joint.

Participants were asked to take a 0.1 gram dose of cannabis. Five hours later, the participants were impaired on complex tasks and felt their driving ability and safety were lowered. However, the participants were able to complete simple tasks similarly to when they were sober. Impairment was found to be the worst three hours after taking the cannabis.

In the study, the 45 participants were aged 18-24 (the age range with the highest percentage of marijuana users in Canada) and identified as recreational cannabis users.

Usage of Marijuana by Canadians of Varying Age and Sex Groups in 2015 (Data from Statistics Canada)

The participants were tested on 4 different days over a 4-6-week period. On those days, they were tested 1, 3, or 5 hours after taking a standardized dose of 0.1 grams of cannabis, or they were tested after taking no cannabis. On days where the participants took the cannabis, they were also tested ten minutes after taking the cannabis.

Ten minutes after taking the drug, participants were asked questions about how they felt about their driving ability and safety as well as how they felt about the effect of the cannabis.

After the wait time of 1, 3, or 5 hours, the participants completed two driving-related tasks, with the two tasks being done in random order. One task was a driving simulation, and the other task asked the participant to identify an object as a car or a truck under three levels of distraction: no distraction, peripheral distraction, and on-screen distraction.

On the day when the participant took no cannabis, the tests were done immediately.

“The message is simple. If you consume, don’t drive. Find another way home or stay where you are,” said Jeff Walker, chief strategy officer for the Canadian Automobile Association (CAA), the organization that funded this study.

Personally, I was quite excited about the legalization of marijuana in Canada, even though I don’t use it. I had heard a lot about its benefits, and that it was harmless after you sobered up again. However, with this study, and questions that have been brought up about roadside testing, I think that it should not have been legalized so soon.

Additional Source: McGill University Newsroom

Footnote:
* The above advice of waiting one hour for each drink is only a guideline. the actual wait time can be higher or lower depending on the person.