Dear Herb: I use cannabis daily. How will I know if I’m above the legal limits for driving?

Pending blood-THC limits in federal Bill C-46 have Canadian cannabis users worried about getting behind the wheel


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Dear Herb: As we move towards implementing new cannabis driving laws/limits, I wonder, how is a heavy (daily) user going to know if they are above the limit?

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Hey there, time traveller!
This article was published 06/06/2018 (1702 days ago), so information in it may no longer be current.

Dear Herb: As we move towards implementing new cannabis driving laws/limits, I wonder, how is a heavy (daily) user going to know if they are above the limit?

I know when I have a beer exactly how much alcohol I have had for my size and what my limit would be, but with cannabis, who knows?

Do you think they will come out with some sort of self-test you could do? — Perturbed by Per Se Limits

Herb answers your questions about legal consumption and growing, the law, etiquette — you name it, he'll look into it.

Dear Perturbed: Thanks for asking. Let’s start with a quick explanation of the forthcoming cannabis-impaired driving law and its proposed limits on THC in the bloodstream, for those who aren’t already familiar.

Bill C-46, introduced in tandem with The Cannabis Act, is partly premised on the idea that cannabis legalization will lead to a spike in marijuana-impaired driving. (C-46 also includes changes to existing laws around alcohol-impaired driving, but we’re here to talk about weed.)

If and when C-46 becomes law, it would give police a new way to establish reasonable grounds that a driver recently consumed cannabis by using an as-yet-unidentified “oral fluid drug screening device” along with current roadside tests. With those reasonable grounds, police could then test for impairment by performing a drug recognition evaluation (which are already in use today) or demanding a blood test (which is new in C-46).

Like the familiar blood-alcohol tests for alcohol-impaired driving, the blood tests will be based on specific limits of THC in the blood.

The problem is, there’s no agreed-upon blood-THC level at which a driver can be objectively considered “impaired” after the fact, because THC is quickly eliminated from the bloodstream. Plus, regular cannabis users could potentially exceed the government’s blood-THC limits even if they aren’t under the influence of cannabis.

(If you want to learn more about the problems with Bill C-46, read The Leaf’s in-depth feature on the subject. It was published in September, 2017 but the information is still accurate as of this writing in 2018.)

Back to your question, Perturbed. How will you, a daily cannabis user, be able to check if you have more than the legal limit of THC in your bloodstream before you hit the road?

As far as I can tell, you won’t. 

Alcohol users can buy little key chain breathalyzers to test themselves against the legal blood-alcohol limit, but the idea of cannabis users performing their own blood tests and lab analyses seems far-fetched.

Once the government identifies its saliva screening device of choice, I suppose it might be possible for Canadian drivers to purchase one for themselves, but those devices will only show whether cannabis has been consumed recently, not whether blood-THC levels are over the legal limit.

However, some people outside government are trying to help cannabis users test themselves for impairment.

The DRUID app, developed by University of Massachusetts psychology professor Michael Milburn, measures impairment by having users perform a series of tasks that measure reaction time, decision making, motor skills, balance and attention. The My Canary app takes a similar approach, testing memory, balance, reaction time, and time perception. (The app’s creators warn that users should not base their decision on whether to drive based on the results of My Canary tests.)

Those digital skill tests could certainly be useful tools for cannabis users, or at least make for fun party games, but as the My Canary disclaimer suggests, those tests fall short of measuring impairment in a way that meets any kind of legal standard.

The government seems to be aware that Canadian drivers will have no way of testing themselves for cannabis impairment. A 2017 FAQ by the Justice Department includes the question, “How much cannabis can I consume before it’s unsafe to drive?”

“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,” the Justice Department said in its answer.

“Mixing driving with cannabis, or any other impairing drug, is not safe and poses a danger to our streets and highways.”

That might be a strong public safety message, but it doesn’t do much to help Canadians who want to stay on the right side of the law.

In conclusion, Perturbed, as far as I know there’s currently no bulletproof self-test to help you know whether or not you’re over the future legal limits on blood-THC levels. If any readers have potential solutions, I’d be happy to hear them.

Got a question about cannabis? Herb answers your questions about legal consumption and growing, the law, etiquette — you name it, he’ll look into it.  Email or to submit anonymously, fill out the form below. Please include an email address if you’d like to be notified when Herb answers your question:  


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