After analyzing blood samples of over 3000 people, researchers at the University of British Columbia found no link between low levels of THC and car crashes.
The study, recently published in the journal Addiction, looked at the impact of cannabis, illegal drugs and alcohol on collisions.
The UBC study analyzed blood samples from more than 3,000 drivers who were treated in B.C. trauma centres between 2010 and 2016. It looked at accident reports for more than 2,300 of those crashes and included 1,178 of those in which the driver was deemed responsible for the crash in its final analysis.
“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, an associate professor at UBC’s emergency medicine department and lead investigator in a five-year study looking at cannabis and crashes.
What’s the legal limit?
“That’s significant because the new impaired driving laws do include penalties for drivers with THC levels between two and five nanograms/ml, suggesting that the laws may be too strict.”
Dr Brubacher has recommended Canada’s laws be changed to not penalize someone with under five nanograms/ml in their blood.
“I am leaning in that direction right now based on this study and the concerns we have raised about habitual users having low levels. A medical marijuana user, for example, would never be allowed to drive.”
After smoking a joint, the levels of THC go up quite high to around 100 nanograms/ml in the minutes after smoking, he said, and then they drop off very quickly.
“In four hours they’d be under two nanograms/ml,” he said.
Alcohol still the biggest factor.
In contrast, the use of cocaine, heroin and alcohol all lead to extremely high increases of accident, with drivers showing a blood-alcohol content over 0.08 six times more likely to crash compared to non-drinkers.
The federal government has a slightly different opinion though, stating that the percentage of Canadian drivers killed in vehicle crashes who test positive for drugs now actually exceeds the numbers who test positive for alcohol.
You can read / download the full UBC study here.