Researchers in both Canada and the United States have questioned the validity of ‘cannabis impairment’ testing. A study from researchers in California in 2023 concluded that, due to the high rate at which the officers determined people from the non-THC group as being impaired, field sobriety testing on its own may not be enough to accurately determine THC impairment.
A 2024 study in Ontario looking at the effects of edible cannabis on simulated driving and blood THC levels found that driving impairment was not correlated with blood THC. The study was the first of its kind to look at the impact of cannabis edibles on simulated driving, with researchers using an average dose of around 7.3 mg THC to provide real-world context for the impact legally available cannabis can have on driving.
Another study in 2024 questioned the accuracy of a method used by police to determine cannabis impairment in Canada. Visual evidence of ‘eyelid tremors’ is one of the processes used by Drug Recognition Experts (DRE) to supposedly confirm if a driver is impaired by cannabis use, including the Royal Canadian Mounted Police (RCMP). Researchers found that there was no significant association between recent cannabis use and the observers’ assessment that eyelid tremor was present. In fact, cannabis users were less likely to have eyelid tremors.
Canadian experts have also questioned the reliability of field sobriety testing conducted by DRE. In Canada, police agencies like the RCMP conduct enforcement of drug-impaired driving using Standard Field Sobriety Testing (SFST) and DRE, in part due to the uncertainty as to the accuracy of THC-detection equipment.
Canada had more than 27,000 people trained in SFST and more than 1,200 active DRE as of the end of 2021. According to another comprehensive study in 2021, there is currently no straightforward way to equate measurements of THC levels in blood or saliva with current driving impairment.
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2 August, 2025