Study: Driving and cannabis

Feb 15, 2022

THC is now being detected in twice as many injured drivers since the legalization of cannabis in 2018, according to the results of a UBC research published in January in the New England Journal of Medicine.

These findings may indicate that increasing numbers of Canadians are choosing to drive after using cannabis. “It’s concerning that we’re seeing such a dramatic increase,” said Dr. Jeffrey Brubacher, the principal author of the study and associate professor in UBC’s department of emergency medicine. “There are serious risks associated with driving after cannabis use. Our findings suggest more is needed to deter this dangerous behaviour in light of legalization.”

As part of the study, researchers analyzed blood samples from 4,339 moderately injured drivers who were treated at four B.C. trauma centres between 2013 and 2020.

Prior to cannabis legalization, 3.8% of drivers had blood THC concentrations above the Canadian legal driving limit of 2 nanograms/ml. However, the study results show that these figures increased to 8.6% after cannabis legalization. Moreover, the study showed that the proportion of drivers with higher concentrations of THC (above 5 nanograms/ml) also increased in recent years, from 1.1% pre-legalization to 3.5% following legalization.

Notably, the most significant increase in drivers testing positive for cannabis use was among drivers over the age of 50. In contrast, there were no significant changes in the numbers of drivers testing positive for alcohol intoxication, either alone or in combination with THC.

Following cannabis legalization in 2018, the federal government amended the Criminal Code, allowing the police to test drivers for drugs when there is reasonable suspicion that the individual in question is intoxicated or has committed a drug-impaired driving offence. Some provinces also introduced new penalties for impaired driving, including fines and driver license suspension.

“The evidence shows that these new laws are not enough to stop everyone from driving after using cannabis,” said Dr. Brubacher in his interview with UBC News. “We hope that policymakers will use our findings to design public information campaigns and enforcement measures that encourage drivers, especially older drivers, to separate cannabis use from driving. At the same time, it is important not to lose sight of alcohol impaired driving, which is extremely high risk, especially when combined with cannabis.”

Dr. Brubacher also added that the presence of THC in the blood is not always an indicator that a collision was caused by cannabis impairment. In fact, in a previous study, Dr. Brubacher and his research team showed that THC concentrations below 5 nanograms/ml do not significantly increase the risk of motor vehicle accidents. However, there is also evidence that higher THC levels of over 5 nanograms/ml significantly increase this risk.

“Detecting cannabis, especially at low concentrations, doesn’t necessarily mean a driver is impaired,” said Dr. Brubacher. “But the risk is real with higher THC levels, which is why it’s so important that we continue to assess and respond to the impact that legalization is having on road safety.”