New Study Suggests Low Levels of THC in Blood Do Not Increase Risk of Car Crash

Canada’s historic legalization of marijuana in October 2018 prompted some major changes to the country’s drug-impaired driving laws. The changes aimed to respond to concerns about legalization’s impact on public safety, especially on the road. As a result, many provinces implemented “zero tolerance” policies for drivers found to have THC in their system. But a new study from researchers at the University of British Columbia suggests Canada’s drug-impaired driving laws may be too strict. According to researchers, the low levels of THC Canada currently considers over the legal limit do not increase drivers’ risk of getting in a car crash.

Researchers: No Link Between Low Levels of THC and Car Crashes

Due to public perceptions about the health and safety risks of legalized cannabis, Canadian officials opted to play it safe. So they set up very conservative legal limits for THC. Under Canadian law, drivers can face fines up to $1,000 for driving with two to five nanograms (ng) of THC per milliliter (mL) of blood. Drivers found to have more than five ng/mL of THC face fines over $1,000 and possible jail time. Canada even has special rules for drivers that mix alcohol and cannabis. Fines above $1,000 and possible jail time await drivers found to have 50 milligrams of alcohol per 100 mL of blood and 2.5 ng/mL of THC in their system.

But a new UBC study suggests the science doesn’t support Canada’s drug-impaired driving laws. According to researchers, the two ng/mL limit is likely too strict, since THC levels up to five ng/mL aren’t associated with higher crash risks.

UBC researchers analyzed blood samples from more than 3,000 drivers treated for automobile accident-related injuries in British Columbia between 2010 and 2016. Researchers also examined more than 2,300 accident reports, including 1,178 reports in which the driver was found responsible for the crash.

Analyzing the blood samples from those drivers who caused crashes, researchers did not find any link between THC levels below five ng/mL and increased risk of accidents. “At blood levels of less than five nanograms/mL, THC does not appear to be associated with an increased risk of crashing,” said Dr. Jeffrey Brubacher, the study’s lead investigator.

Drugs, Alcohol Still More Dangerous for Drivers than Cannabis

Accurately measuring someone’s impairment from THC is extremely challenging. Unlike alcohol, which has a fairly direct relationship between blood content, body weight and intoxication, the presence of THC in the blood doesn’t necessarily signal impairment. Furthermore, the complex metabolic pathway of THC makes it difficult to judge how long a cannabis consumer should wait before getting behind the wheel. In short, there’s no obvious relationship between having THC in your system and driving high.

But there is a significant relationship between drugs like cocaine, amphetamines, and heroin and increased risks for automobile crashes. Compared to THC, the UBC study found that hard drugs increased the risk of a crash by 82 percent. Furthermore, common prescription drugs and over-the-counter medication like antihistamines and antidepressants increased crash risks by 45 percent. But researchers found alcohol to have the highest association with crash risk. Drivers with a blood-alcohol content over 0.08 were 600 percent more likely than non-drinkers to crash their vehicle.

Importantly, researchers found no comparable association between THC levels below five ng/mL and increased crash risk. They study even casts doubt on whether THC levels above the five ng/mL level increase accident risks. Of the 1,825 blood samples researchers analyzed, just 20 had THC levels above five ng/mL.

In short, alcohol, common legal medications and hard drugs are way more risky for drivers than THC. “The biggest problem remains alcohol,” said Dr. Brubacher. “Drug-impaired driving is probably a problem, but let’s not lose sight of alcohol.”

Will Canada Revise “Driving High” Laws in Light of New Research?

The UBC study on drug-impaired driving shows that Canada’s laws against driving high may be overly harsh for cannabis consumers. Drivers are facing fines, revoked drivers licenses and possible jail time for THC levels that aren’t causing more accidents or making roads less safe. The UBC study seems to corroborate other reports and surveys that say legalization isn’t increasing incidents of drug-impaired driving.

So will the UBC study prompt Canadian officials to change their drug-impaired driving laws? For now, law enforcement and public safety officials are sticking to the current limits. Canada’s federal government is maintaining that cannabis increases the risk of car crashes. Police remain convinced that cannabis is impairing and that the restrictive limits make sense.

At the same time, law enforcement and policymakers are welcoming further research into how cannabis consumption impacts traffic safety. “And I think a lot more research needs to be done before we’re willing to change perspective,” said Chair of the Canadian Association of Chiefs of Police Drug Advisory Committee Mike Serr.