The skinny on cannabis and impairment
Updated: Sep 28
In the last three decades, Canada and the U.S. have seen a pronounced drop in alcohol-impaired driving. The social acceptability of driving drunk, or allowing others to drive drunk, has changed dramatically. Alcohol’s effects on the ability to operate a vehicle are no longer up for debate. Alcohol impairs driving skills, and impaired driving can have deadly consequences.
Decades of research, regulation and public education allowed governments to establish legal blood-alcohol limits, develop reliable testing methods and introduce programs discouraging risky behaviours. Over time, generations of drivers grew up understanding the risks of getting behind the wheel after drinking.
Cannabis has followed a different path. Its acceptance and regulation have changed significantly, particularly since legalization in Canada in 2018. As cannabis has become normalized, our understanding of cannabis-impaired driving, roadside detection and the relationship between THC levels and impairment has continued to evolve.
The good news is that impaired driving overall has declined considerably from historical levels. Police reported 71,602 impaired-driving incidents in Canada in 2023, representing the lowest rate recorded. Alcohol remained the most common substance involved, accounting for 75% of police-reported impaired-driving incidents. Drug-impaired driving alone accounted for 8.5%, or 6,092 incidents, a slight increase from 5,935 incidents in 2022.
Understanding these numbers is important because alcohol and cannabis do not affect the body in exactly the same way, nor can impairment from the two substances be measured in precisely the same way.
THC and driving
Cannabis can impair the skills required for safe driving. THC can affect attention, coordination, reaction time, decision-making and the ability to judge distances.
However, equating cannabis impairment directly with alcohol impairment is problematic. Ethanol and THC are different molecules, behave differently in the body and can affect driving behaviour differently.
Driving simulator research has historically found differences in the behaviours associated with alcohol and cannabis impairment. Some cannabis-impaired drivers may reduce their speed or attempt to compensate for feeling impaired, but compensating does not make driving safe. Slower reaction times, reduced attention, impaired coordination and altered perception can still interfere with a driver's ability to respond to changing road conditions.
This is also why a person's perception of whether they "feel high" is not a reliable measure of whether they should drive. Cannabis affects people differently depending on factors including dose, THC concentration, method of consumption, frequency of use and individual physiology.
Understanding Canada’s THC limits
Canada currently has specific Criminal Code offences based on the concentration of THC in a driver's blood within two hours of driving.
There are three important thresholds:
2 ng/mL to less than 5 ng/mL of THC: This is a summary conviction offence based on a precautionary approach.
5 ng/mL or more of THC: This is a more serious hybrid offence and is more closely associated with recent cannabis use and impairment.
2.5 ng/mL or more of THC combined with 50 mg or more of alcohol per 100 mL of blood: This is also a hybrid offence and specifically recognizes the increased impairment associated with combining cannabis and alcohol.
These limits should not be interpreted in the same way as a blood-alcohol concentration limit. THC is highly lipophilic, meaning it is attracted to and stored in fatty tissues. Its concentration in blood can rise rapidly after cannabis is inhaled and then decline substantially, while THC metabolites can remain detectable much longer.
This creates an important distinction between detecting cannabinoids and determining current impairment. Cannabis metabolites can remain in the body long after the intoxicating effects have subsided, particularly among frequent cannabis consumers.
For that reason, there is no simple formula that tells a person exactly how much cannabis they can consume or precisely how long they need to wait before it is safe to drive.
The Government of British Columbia states that there is no standard or safe amount of time to wait before driving after using cannabis. Federal guidance similarly notes that impairment varies depending on the person, product, dose and method of consumption.
Canada's Lower-Risk Cannabis Use Guidelines recommend waiting at least six hours after using cannabis before driving, but even that should not be treated as a guarantee that someone is no longer impaired. Some effects can last considerably longer, particularly following higher doses or edible cannabis.
The safest approach is straightforward: if you are impaired or uncertain whether you are impaired, don't drive.
How police test for cannabis
Roadside enforcement has also changed considerably. Police continue to use observational and behavioural tools, including Standard Field Sobriety Tests and Drug Recognition Expert evaluations. These assessments can consider physical coordination, behaviour and other signs associated with drug use or impairment.
However, police are no longer limited to observational criteria. If an officer has reasonable grounds to suspect that a driver has a drug in their body, they can demand an oral fluid sample using approved drug-screening equipment. These devices can detect the presence of certain drugs, including THC. A positive oral-fluid result, combined with other evidence observed by police, can provide grounds for further investigation, including a demand for a blood sample.
This is an important distinction from alcohol screening. Police can demand a preliminary breath sample from a lawfully stopped driver without first developing reasonable suspicion that alcohol is present. For an oral-fluid drug-screening demand, reasonable suspicion that a drug is in the driver's body is required.
The problem with mixing substances
Cannabis and alcohol deserve particular attention because their combined effects can be greater than either substance alone.
Health Canada warns that combining alcohol with cannabis can significantly increase impairment. Research examining their combined effects has found greater impairment in areas such as reaction time and vehicle control compared with either substance alone.
Canadian law explicitly recognizes this additional risk. A driver who has 2.5 ng/mL or more of THC in their blood combined with 50 mg or more of alcohol per 100 mL of blood within two hours of driving can face a hybrid criminal offence.
This means that being below the 5 ng/mL THC threshold does not make combining cannabis and alcohol legally or practically safe.
The same caution applies when cannabis is combined with other psychoactive substances. Sedating medications, opioids, benzodiazepines and other drugs can compound impairment and further reduce a person's ability to react, concentrate and make decisions behind the wheel.
Edible cannabis introduces another consideration because its effects can take longer to appear and can last much longer than inhaled cannabis. Health Canada notes that it can take up to two hours to begin feeling the effects of edible cannabis, up to four hours to feel the full effects and as long as 12 hours for acute effects to subside. This makes judging impairment based simply on how someone feels shortly after consumption particularly unreliable.
Education remains essential
Education played an important role in changing Canadian attitudes toward alcohol-impaired driving. Public awareness campaigns, advocacy and education helped establish the understanding that driving after drinking creates risks not only for the driver but for everyone sharing the road.
Cannabis requires the same commitment to evidence-based education.
The challenge is that cannabis impairment cannot simply be treated as a mirror image of alcohol impairment. THC behaves differently in the body, individual responses vary considerably and there is no universally applicable formula telling someone exactly how much cannabis they can consume or how long they must wait before driving.
What is clear is that cannabis can impair driving, combining cannabis with alcohol increases impairment, and Canada has established specific THC and combined THC-alcohol limits under the Criminal Code.
As cannabis continues to be normalized, education needs to evolve alongside the science. Consumers need accurate information about impairment, administrators and regulators need policies grounded in current evidence, and everyone needs to understand one simple principle: if cannabis or another substance is affecting your ability to drive safely, don't get behind the wheel.
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If you have questions about harm reduction, safe usage or anything else under the banner of cannabis, book a virtual appointment with Dr. Ife Abiola.
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