Article by Michael Barclay, Macleans.ca
Nobody knows. At the very least, you should wait two hours after one hit, if not four hours or more—and that’s highly dependent on the THC level of the joint. You should wait even longer after you gobble an edible.
The easy answer is: don’t toke and drive. Especially in Quebec—there, you’ll get a 90-day licence suspension if the police find any at all in your system. (Most other provinces dole out the same punishment only if a driver’s THC level is above the legal limit.)
A 2016 survey by State Farm Insurance found 44 per cent of Canadians didn’t think cannabis affected driving skills. Lots of your stoner buddies will tell you they drive better while high—that they’re more “focused.” But the problem is that stoned drivers can be too focused: on their speed, on the car ahead, on any one particular aspect of their driving, and they might not notice a sudden change on the road ahead.
Traces of THC can remain in your system for up to 12 hours after consumption, whether you’re feeling it or not. (This will definitely be a problem for medical users.) There is no agreement on how much THC is too much to drive—the individual effect is far too subjective. First-time users will be affected much more than long-time tokers. The American Automobile Association’s Foundation for Traffic Safety declared in a 2015 report that there is no blood-concentration level of cannabis that would provide an accurate barometer of someone’s impairment level.