Canadian studies suggest THC levels may not measure marijuana impairment accurately 

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  • Mitch M / Shutterstock


State of Michigan officials recently chose not to set a per se level of THC in blood or saliva to constitute impairment when driving. As more results on the subject come in, it looks like the state Impaired Driving Safety commission made the right recommendation.



Our friends to the north who seem to have more experience considering cannabis without freaking out have recently published two different studies that suggest a more nuanced approach to assessing impairment when driving after imbibing.

The province of Ontario published a review of literature in the journal Public Health concluding that roadside oral fluid tests are simply not sensitive or accurate enough to judge THC levels. In addition, it pointed out, we don't know the "correlations between THC concentrations in OF (oral fluid) and the level of impairment."

Folks at the University of British Columbia added to the information, with the results of a six-year study in the journal Addiction on the likelihood of crash responsibility by drivers who tested positive for intoxicating substances including THC.

"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," researchers concluded. "For drivers with THC>5ng/ml 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."

The UBC study also found that there was significantly increased risk for drivers who used alcohol, sedating medications, and recreational drugs other than cannabis. Drivers who used alcohol and cannabis together had a risk much higher than alcohol alone. Anybody who has used marijuana and alcohol at the same time probably knows that there seems to be a multiplier effect when you mix the two that takes one well beyond where either of those substances get you on their own.

The risk of traffic accidents is one of the biggest arguments when a state is considering legalizing marijuana. Law enforcement is particularly concerned with the issue. Each time information like this comes out, it takes the hysteria down a peg. That's a good thing.

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