A new study is questioning the accuracy of a method used by police to determine cannabis impairment. 

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 RCMP.

“Eyelid tremor” is a term that refers to involuntary and intermittent spasms of the eyelid muscles and has often been associated with cannabis impairment. It’s identified under the Drug Evaluation and Classification Program of the International Association of Chiefs of Police. 

In Canada, cannabis impairment is often determined via visual inspection by Drug Recognition Experts trained in detecting impairment as part of a multi-step process that includes an examination of a person’s eyes. 

The objectives of this study were to assess the accuracy and replicability of identifying eyelid tremors as an indicator of recent cannabis smoking using a blinded, controlled study design.

About 100 people were recruited to participate in the study and divided into three categories: daily, occasional, and no current cannabis use. 

Participants then had their closed eyelids video recorded for 30 seconds by infrared videography goggles before and after smoking or vaping cannabis. 

Three observers with experience in movements of the eye and medical toxicology then reviewed these videos to determine which individuals had consumed or were impaired by cannabis based on a grading system. 

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.

Because of this, the study says that the weak association between recent cannabis use and eyelid tremor does not support this method in identifying recent cannabis use.

The study concludes that additional research is needed to identify the presence of eyelid tremor more accurately. Furthermore, research should determine the relationship, if any, between cannabis dose and timeline in relation to last cannabis use to eyelid tremor and how, or if, it should be used for cannabis Drug Recognition Evaluator examinations.

A study from researchers in California in 2023 also 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.

Canadian experts have also questioned the reliability of field sobriety testing conducted by  Drug Recognition Experts (DREs). In Canada, police agencies like the RCMP conduct enforcement of drug-impaired driving using Standard Field Sobriety Testing (SFST) and DREs, 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 DREs 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 and current driving impairment. 

A 2021 report from Quebec based on coroners’ and medical examiners’ report data from toxicological analyses found that when cannabis was present, 20 percent had a THC blood concentration between 0.5 and 1.9 ng/mL, 44% between 2.0 and 4.9 ng/mL, and 36 percent over 5 ng/mL. Canada’s prohibited blood drug concentration (BDC) level is between 2 ng and 5 ng THC per ml of blood. 

A THC rate below 2 ng/ml does not necessarily indicate recent use, while a rate over 5 ng/ml is associated with impaired driving and recent use. In addition, a rate over 5 ng/ml increases the likelihood of an impairment-responsible crash. Polydrug use was found in 79% of cases.

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