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The Risks of Mixing Driving and Cannabis

THC, like alcohol, does impair driving ability, and its effects are not reversed by CBD, as some people claim.

This article was first published in The Montreal Gazette.

In any other year, we would be reminding people right now about the dangers of drunk driving after their office Christmas parties and family get-togethers. Hopefully, we won’t see many parties this year, but the dangers of impaired driving remain. And while alcohol is the major culprit in impaired driving accidents and fatalities, other recreational drugs also play a role.

After alcohol, cannabis is the most commonly detected drug in motor vehicle accidents, and the incidence seems to be rising.

The problem in linking cannabis with traffic accidents is that when people test positive for it after a car crash, they can often test positive for alcohol and other drugs too, which confuses the association. Also, blood levels of cannabis’s main active ingredient, THC, do not necessarily correlate with your degree of intoxication in the same way that alcohol does. The higher your blood alcohol level, the more impaired you are. But with THC, the relationship is less clear cut. Also, the effect may vary based on the relative concentration of THC versus CBD, because THC is, generally speaking, the psychoactive ingredient that causes the symptoms of a high.

Testing the effects of THC versus CBD on driving impairment would be difficult to do in a real-world testing situation. And yet researchers in the Netherlands performed just such a study and published the results last week in JAMA. They took 26 healthy volunteers and randomized them to vape THC, CBD, a THC/CBD mix or placebo and then had them do a driving test after 40 minutes and then after four hours.

Amazingly, they did the driving tests on an open highway rather than on a closed course or in a driving simulator. As many colleagues have pointed out, you would probably not get ethics approval to try a similar experiment in Canada or the United States.

There were, mercifully, no accidents though 16 driving tests were terminated because of safety concerns and participants who felt they were too high to drive could opt-out.

Study subjects were told to drive straight along a continuous highway and stay in the same lane. Driving impairment was captured by a rooftop camera that measured how much the driver swerved laterally during driving.

By the four-hour driving test, the effects of the cannabis products had largely worn off and none of the three groups differed from placebo. However, at the 40-minute test, after vaping THC or the THC/CBD combo, drivers showed more lateral swerving in their driving. Interestingly, there was no difference between the CBD and placebo groups.

The one criticism of the study was that the doses of THC and CBD used were very small. A follow-up study using larger doses is apparently underway, though wisely this will apparently be done in a driving simulator. Also, since particularly unsafe driving tests were stopped early or canceled, this likely skewed the results and made the vaped cannabis seem safer than it might otherwise be.

Still, though, this study makes a few important points. Cannabis, though legal, is not benign. THC, like alcohol, does impair driving ability, and its effects are not reversed by CBD, as some people claim. Cannabis that has mostly CBD with very little or no THC may have a lesser impact on driving skill, though given the relative uncertainty on the issue, it would probably be better not to take the risk.

Suffice it to say, there is more and more evidence that driving while high is just as problematic as driving while drunk. People may choose to indulge in the recreational use of cannabis and there may be some medical applications, but that should not lull us into thinking the product is without risk, particularly when you get behind the wheel.


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