The shortest version: in Canada, you can be charged with impaired driving even if you're well under any THC blood limit, and you can be charged on the limit alone even if you're functionally fine. Both things are true, and both things are why the answer is "don't drive."
What the law says
The federal Cannabis Act and the Criminal Code created three offences for THC behind the wheel:
| Blood THC level | Offence |
|---|---|
| 2–5 ng/mL | Summary offence ("less serious") |
| 5+ ng/mL | Hybrid offence |
| Any THC + 50 mg/100 mL of alcohol | Hybrid offence |
A "hybrid offence" means the Crown can pursue it as either a summary or indictable charge — which translates to "much heavier penalties available."
Beyond the per se limits, impairment itself is a separate offence. Police don't need a THC reading at all if a Standardised Field Sobriety Test (SFST) or Drug Recognition Evaluation (DRE) supports the call.
Provinces stack on top:
- Zero tolerance for novice + young drivers (under 21 in some provinces, under 22 in BC, all G1/G2 in Ontario). Any detectable THC = roadside suspension.
- 24-hour and 90-day administrative suspensions at the provincial level happen before any criminal trial.
How long should I wait?
Health Canada's guidance for an occasional user is at least 6 hours after smoking or vaping, and at least 8 hours after edibles. That number assumes a typical dose and a sober baseline; the actual half-life of THC in blood is long and variable, especially for frequent users.
For frequent users (daily for weeks/months), blood THC can stay over 5 ng/mL for days with no felt impairment. The per se limits aren't kind to that scenario. The honest answer is: if you use cannabis daily, you probably can't reliably drive in compliance with the per se law at all. Plan around transit or a sober driver.
Why roadside testing is shaky
Saliva swabs (e.g. Dräger DrugTest 5000) measure recent oral cannabinoid exposure, not blood THC or impairment. The correlation between oral fluid and actual driving impairment is weak — recent reviews suggest false-positive and false-negative rates that would be unacceptable in clinical contexts.
In practice this means:
- A positive saliva test gives the officer grounds to demand a blood draw at the station.
- A negative saliva test doesn't necessarily mean you're under the per se limit.
- Either way, the SFST + DRE pathway is the one that produces a conviction.
The takeaway
Cannabis impairs reaction time, lane-tracking, and decision-making — measurably, even at modest doses, and especially when combined with alcohol or sleep deprivation. The Cannabis Act exists; the courts will enforce it; the science of roadside detection is still catching up.
Don't drive. Use the time to listen to the album back-to-front like you used to.