The Canadian government has approved a roadside test for measuring THC levels in a driver’s system. There are some critical problems with this new device.

THC levels cannot indicate impairment.

Let me get all sciency for a minute:
Your body’s endo-cannabinoid system produces endo-cannabinoids to interact with your brain to help manage things like mood and healing. When you use cannabis, you are supplementing that existing process inside your body. The cannabis plant has hundreds of active compounds that include cannabinoids, as well as terpenes and other chemicals that all interact with each other. Since a terpene is simply a smelly plant—based compound, even the foods you eat can interact with the cannabinoids and alter their effects.

It is impossible to predict the effects of THC on an individual’s cognitive ability without understanding all of the bio-chemistry. And since marijuana research was effectively banned for decades, nobody today really understands it. This machine only measures THC, no other cannabinoids, much less collecting a food history or providing a lifestyle analysis, so it does not even provide insight into the biochemistry on a case by case basis.

THC remains in the body long after cannabis use

Alcohol breaks down relatively quickly in the body. Within 24 hours after stopping use, most of the alcohol has dissipated. While scientists can see the effects of alcohol on the brain up to a week later, the alcohol itself is gone.

THC breaks down very slowly in the body. At one time, the “rule of thumb” was 30 days to dissipate completely, but this varies from person to person. An occasional, recreational user should be compliant with Canada’s zero-tolerance legislation about 2 weeks after use.

The law unfairly targets medicinal users

Medicinal users are practiced at balancing the cannabinoids to cancel out the effects of THC. THC’s cognitive effects are greatest when there is a significant change in the body’s THC levels. Consistent use of medication effectively eliminates these changes. Furthermore, it takes longer to flush the larger amount of THC from the body, so there is an even longer waiting period to meet the zero-tolerance requirement.

Medicinal users of cannabis consume more THC more often, so they will always test high, despite their being the least affected by the THC. I would recommend waiting at least 2 months after your last dose of medication before you drive. Some provinces (AB, MN) have created driver’s license endorsements for cannabis users with a valid prescription in recognition of this. It’s time for other provinces to follow suit.