This is one of the most common questions I am asked from cannabis users who are afraid of cannabis-impaired driving charges. Most people want to know if there is a certain window they should wait from the time they last smoked before driving, so there is no sign of impairment.
And while the question has never had a good answer scientifically, given that the absorption and elimination rates of cannabis from the bloodstream are non-linear, the question has become even more difficult to answer as a result of a recent study.
A comprehensive review by DeGregorio et al. (2024) demonstrates that the time since cannabis use may not accurately reflect impairment. The study also addresses why relying solely on THC levels in testing may lead to false conclusions about a person’s level of impairment. We have discussed this phenomenon on the blog earlier this week.
As we’ve discussed, a positive THC result in oral fluid or blood doesn’t necessarily mean that a person’s ability to operate a motor vehicle is impaired by it. Unsurprisingly, many THC-positive drivers aren’t impaired, either because they weren’t impaired shortly after using cannabis or because THC can linger in the body for a long time after it was used.
Common testing methods focus on detecting THC, which can persist in bodily fluids long after the effects of cannabis have worn off. This leads to a host of concerning issues demonstrating the really problematic nature of using this type of testing evidence in court and for punishments under administrative regimes, like a 24-hour prohibition for drugs or a 90-day Administrative Driving Prohibition for drug-impaired driving.
In some studies, THC was detected in saliva samples up to 61 hours after smoking. This happened in frequent users. It was also shown in studies as being present in the blood after 30 days of abstinence from cannabis products! Obviously this is a clear indication that THC in the blood or saliva is simply not evidence of impairment.
But there’s more. There is also no straightforward relationship between THC concentration and impairment. Setting THC concentration thresholds to define impairment lacks scientific justification. Recent studies have shown no direct link between THC concentrations and impairment, rendering THC legal limits scientifically unsound.
Additionally, the combined use of cannabis and alcohol doesn’t necessarily increase impairment or crash risk beyond what would be expected from alcohol use alone. While THC can affect certain driving performance measures, combining cannabis with alcohol doesn’t significantly impact overall driving performance or crash risk compared to alcohol alone. In Canada, we have different concentration thresholds for criminal charges when these two substances are combined, but that probably is not good science-based legal policy.
Because the time since you last used cannabis and what your blood or saliva may show is not directly related, the time since cannabis use and THC levels alone may not accurately reflect impairment. This means that there is no clear answer to how long you should wait after driving. For some people that answer may be 61 hours or as long as 30 days. That’s obviously not going to work for people who need to rely on their vehicles to work and run their lives.
At the end of the day, this is a question that has no proper answer, and there is nothing a lawyer can say to safely advise a person who uses cannabis how to avoid criminal charges for impaired driving.
