By Andrea Gunn

There are many different answers out there about how long one should wait after smoking up before its safe to get behind the wheel, ranging from four hours to a full day.

Health Canada says that, unlike alcohol, the existing scientific evidence does not provide general guidance to drivers about how much cannabis can be consumed before it is unsafe to drive, or how long a driver should wait to drive after consuming cannabis. Health Canada warns that mixing driving with the drug is not safe and that impairment can last up to 24 hours.

Under the new impaired driving legislation set out in Bill C-46, having more than two nanograms of THC per millilitre of blood within two hours of driving can result in a fine, and more than five nanograms can result in criminal charges.

How long the effects of marijuana will last in any given person depends on a number of different factors including weight, experience level and how the cannabis is ingested.

Andrew Murie, CEO of MADD Canada, said that while the often-cited guidelines of waiting a minimum of four hours but up to eight are generally a good starting point, they won’t work for everyone.

“If you mix with alcohol those time limits don’t apply, if you’re a new user, they don’t apply. If you’re a constant regular user you should assess the situation after four hours: if you’re feeling good and you’re no longer high you are probably okay to drive, but if you’re still feeling the effects you probably need to wait six hours,” he said.

“There’s no magical number out there.”

If you’re ingesting marijuana orally, the effects of the THC take much longer to hit and last much longer than smoking or using a vaporizer, Murie said.

“Eating cannabis is like drinking alcohol: you need to take the night off, you can’t consider driving.”

Mike Serr, deputy chief of the Abbotsford, B.C., police department and drug advisory committee chair with the Canadian Association of Chiefs of Police, said the most commonly cited minimum time to wait after smoking cannabis before driving is four hours, but he too warned this does not apply universally.

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