The problem with field sobriety tests in assessing marijuana impairment

On Behalf of | May 26, 2022 | Drug Crimes, Impaired Driving

Recreational marijuana use has been legal in Canada since 2018. However, driving under the influence of the drug is still illegal – and can result in stiff penalties.

When it comes to marijuana, there is no hard and fast test for how police can measure impairment. So, how do they determine whether you are unfit to drive based on marijuana influence?

Alcohol testing is more straightforward

With alcohol, police have breathalysers and blood tests that quickly determine when drivers are too intoxicated to be behind a wheel. They also have years of research on the signs of alcohol intoxication – such as slurred speech or memory loss. But testing for alcohol and marijuana can be very different.

Challenges with marijuana testing

With marijuana, officers may ask to do a blood test. Currently, Ontario’s legal limit is five nanograms per millilitre of active tetrahydrocannabinol (THC) in the blood. Since THC can take time to leave the body, these tests can often be inaccurate for frequent users, like those who use marijuana for medicinal purposes.


Beyond testing for THC, much of the decision rests solely on the officer’s judgement. They may look for signs of marijuana intoxication or for paraphernalia. They may also administer a field sobriety test, similar to the one used for alcohol. Unlike alcohol, however, there is not a lot of research on when someone is too impaired by marijuana to drive. Often this testing can come into question in court.

Due to the challenges associated with testing, impaired driving offences for marijuana tend to be more subjective to the police officer who arrested you. Being a frequent user may make charges more likely. Until further research can give police better tests, it is important for you to protect yourself from a mark on your criminal record. A criminal defence lawyer experienced in impaired driving cases can help.