Understanding Cannabis and Driving: The Road to Improved Regulations

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As more US states and entire nations are establishing medical and adult-use cannabis programs, the need for updated and cohesive cannabis driving laws is imperative.

In the past 10 years as legalization has spread at a rapid rate, researchers are racing to assess the impacts of cannabis on driving. Yet much like the varied pharmacological effects of cannabis, understanding how and to what extent cannabis impairs driving is complicated.

Current roadside drug testing is conducted by various means throughout the world. The most common testing methods include:

  • Tongue scrape
  • Saliva/Oral fluid sample
  • Plasma or blood test


There are benefits and concerns across these methods. Tongue scrapings aren’t always accurate, they often only have the capacity to indicate cannabis, not the presence or concentration of THC specifically.

Saliva samples are the most common confirmation used in roadside drug testing. Despite giving a greater picture of THC levels, they don’t account for variations in tolerance, time or method of ingestion. These factors can greatly influence the magnitude of impairment on driving.

Blood tests that provide full toxicology report of cannabinoids and their metabolites are the best way to determine the time of subjective high and probable impairment. Unlike blood alcohol, the levels of THC present in saliva or blood aren’t a definitive determinant of cognitive impairment.


Where blood alcohol levels can be directly correlated with levels of impairment and extrapolated back in time, this is not the case for cannabis. Cannabis is metabolized at different rates by different individuals, therefore a measure of blood cannabinoid levels including metabolites of degraded THC is a more accurate measure of the time of use and subjective high.

THC plasma concentration, performance deficit and subjective high after smoking cannabis

Tolerance, the method of ingestion and frequency of use are also variables that need to be accounted for. Whether cannabis is ingested as an edible, smoked or otherwise has a dramatic influence on how long it is present in body fluids and at what time psychoactive levels peak. In terms of regular versus occasional consumers, drug tests have shown positive THC levels for anywhere between 48 hours to 30 days after ingestion. While occasional users tend to show little, if not no THC left after 48 hours.

For long term medical cannabis and legal adult consumers, in cases where they are driving when sober, this presents a problem. As THC is stored in body fat tissue, it can be released into the body over a long period of time. For places that have cannabis regulations set by per se cannabis levels, it is possible for long term users and patients to exceed limits even during times of sober driving or prolonged abstinence.

Standardized roadside sobriety tests are still in place in many regions of Canada and the US. Such methods employ walking in a straight line, standing on one leg amongst other physical tests to determine the level of motor deficit. While such testing has been long used for confirmation of alcohol or drug inebriation, they are still quite subjective measures and interpretation is at the behest of the officer.

When it comes to cannabis driving impairment, as yet there seems to be no superior stand-alone parameter. Moving forward, further investigation into standardized measures need to be fairly determined. Recommendations from various reports and research recommend not basing impairment on per se levels or blood plasma testing and training law enforcement officers on an accurate assessment of active THC impairment.

Studies suggest trained Drug Recognition Experts have a strong ability to recognize impaired drivers. By and large, it seems the best way to navigate the increase of cannabis affected drivers on the road is a combination of assessments by trained experts and sensitive THC testing devices.


Driving experiments and randomized clinical trials have shown that THC does impact abilities in both simulated and real driving. However, variations in dose and consumer type (medical, chronic or acute consumers) suggest that impairment can be affected in a dose-dependent manner, as well as based on the frequency of prior cannabis use.


Finely-tuned and specific markers of behavioral and motor deficits will have to be developed. This will differentiate between unsafe cannabis influenced driving and typical levels of THC blood concentration in medical cannabis users who are responsible and unimpaired.

It is clear that cannabis impacts driving ability in many instances and current understanding of cannabis effects do not support per se levels as the only measure of sobriety. Models that law enforcement are acquainted to (where blood alcohol levels correlate more directly with impaired driving) will have to be replaced in the face of widespread cannabis reform.

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