Presence Of THC In Blood is Not Associated With Crash Culpability, Finds Study

According to a new study, drivers who test positive for THC do not possess a significantly increased risk of being responsible for a non-fatal motor vehicle accident.

For the study, researchers at the University of British Columbia compared the likelihood of crash responsibility in drivers testing positive for THC and/or other substances as compared to drug-free drivers over a six-year period (2010 to 2016).

As reported on by NORML, the state found that, “In this multi-site observational study of non-fatally injured drivers, we found no increase in crash risk, after adjustment for age, sex, and use of other impairing substances, in drivers with THC<5ng/ml. For drivers with THC>5ngml there may be an increased risk of crash responsibility, but this result was statistically non-significant and further study is required. … Our findings … suggest that the impact of cannabis on road safety is relatively small at present time.”

By contrast, authors reported, “There was a significantly increased risk for drivers who used alcohol, sedating medications, or recreational drugs others than cannabis.” Drivers who tested positive for the concurrent use of cannabis and alcohol possessed a higher risk of accident as compared to drivers who tested positive for alcohol alone – a finding that is consistent with other studies.

The full text of the study, titled “Cannabis use as a risk factor for causing motor vehicle crashes: A prospective study,” has been published in the journal Addiction.

The full abstract of the study can be found below:


We conducted a responsibility analysis to determine whether drivers injured in motor vehicle collisions who test positive for Δ-9-tetrahydrocannabinol (THC) or other drugs are more likely to have contributed to the crash than those who test negative.


Prospective case-control study.


Trauma centres in British Columbia, Canada.


Injured drivers who required blood tests for clinical purposes following a motor vehicle collision.


Excess whole blood remaining after clinical use was obtained and broad spectrum toxicology testing performed. The analysis quantified alcohol and THCand gave semi-quantitative levels of other impairing drugs and medications. Police crash reports were analyzed to determine which drivers contributed to the crash (responsible) and which were “innocently involved” (non-responsible). We used unconditional logistic regression to determine the likelihood (Odds Ratio) of crash responsibility in drivers with 0<THC<2ng/mL, 2ng/mL≤THC<5ng/mL, and THC≥5 ng/mL (all versus THC=0 ng/mL). Risk estimates were adjusted for age, sex, and presence of other impairing substances.


We obtained toxicology results on 3005 injured drivers and police reports on 2318. Alcohol was detected in 14.4% of drivers, THC in 8.3%, other drugs in 8.9% and sedating medications in 19.8%. There was no increased risk of crash responsibility in drivers with THC<2ng/mL or 2≤THC<5ng/mL. In drivers with THC≥5ng/mL, the adjusted OR was 1.74 (95%CI=0.59-6.36;p=0.35). There was significantly increased risk of crash responsibility in drivers with BAC≥0.08% (OR=6.00;95%CI=3.87-9.75;p<0.01), other recreational drugs detected (OR=1.82;95%CI=1.21-2.80;p<0.01), or sedating medications detected (OR=1.45;95%CI=1.11-1.91;p<0.01).


In this sample of non-fatally injured motor vehicle drivers in British Columbia, Canada, there was no evidence of increased crash risk in drivers with THC<5ng/mL and a statistically non-significant increased risk of crash responsibility (OR=1.74) in drivers with THC≥5ng/mL.

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