As Canada prepares for marijuana legalization, MP Bill Blair is pushing for the introduction of saliva tests to detect drivers impaired by the drug.

A saliva test consists of a small plastic stick that is swiped over a driver's tongue if they're suspected of driving under the influence. The saliva mixes the enzymes in the device: if there's cannabis in their system, the test displays a red line.

"If a jurisdiction was to put in place a strict prohibition on using marijuana for example in driving, that could result, if there is the presence of marijuana metabolites, in the suspension of licence and seizing of a car," said Blair in a recent interview with CBC. "(That's) an immediate consequence which can have a very effective deterrent impact."

Similar saliva tests are current in use in some European jursidictions.

Benefits of testing saliva

The tests, unlike urine tests, are very difficult to adulterate. According to the manufacturers of one popular test, NarcoCheck, the stick detects the "Δ9-THC" - a molecule of cannabis found specifically in the mouth for several hours after someone smokes a joint.

"Whatever the test used," according to NarcoCheck, "THC can never be detected more than 4 to 6 hours in saliva [...] saliva test[s] fully exploit this window of time."

Salvia tests are ideal for roadside use because they determine quickly whether the person being tested has smoked within the past few days or even hours - unlike urine or hair tests, which can detect use going back as far as several months.

Challenges remain

Like other proposed roadside testing devices, however, saliva tests have several significant flaws: for one, they don't actually measure a driver's level of intoxication.

"In Canada we have not established a baseline of impairment. So we need to speak to the scientists and we need to look at the example in other jurisdictions," Blair tells CBC.

As well, according to one 2006 study published in The Clinical Biochemist Reviews, roadside salvia collection "may be thwarted by lack of available fluid due to a range of physiological factors, including drug use itself. Food and techniques designed to stimulate production of oral fluid can also affect the concentration of drugs."

While devices like NarcoCheck are specifically designed to detect recent cannabis smoking, it's difficult to say how different consumption methods, including vaporizing and edibles, might impact the test's accuracy.

But it's critical to figure out an accurate roadside test of some kind. According to a recent report by State Farm, almost half of Canadian drivers who consume cannabis - that's 44 percent - say that they don't think being high affects their ability to drive.

    h/t CBC News.