Disclaimer: This post presents data from a research paper and is for educational purposes only. The following does not constitute advice as to whether a person should or should not drive and should not be used as a substitute for advice from an appropriately qualified clinician and/or the Driver and Vehicle Licensing Agency (DVLA).
While blood tests and breathalyser analyses have been used for decades to detect alcohol-related impairment in drivers. These methods have also been controversially applied to the use of cannabis; however, the results of a recent study suggest that these tests may not be a consistent indicator of impairment related to cannabis or tetrahydrocannabinol (THC).
As a growing number of countries and jurisdictions continue to increase access to cannabis, including for both medical and recreational consumption, it is likely that a more accurate measure of THC impairment will be needed. Currently, there are three methods used to identify cannabis-impaired drivers – ‘effect-based’, ‘zero-tolerance’, and ‘per se’.
The ‘effect-based’ method involves testing for functional impairment (e.g., field sobriety tests), while ‘zero-tolerance’ and the ‘per se’ methods are both based on the amount of THC present in blood and other samples (e.g saliva). The zero-tolerance approach does not allow the detection of any amount of THC in drivers. In comparison, the per se approach prohibits driving at or above a pre-defined concentration, similar to a legal alcohol concentration limit.
A recent review used driving and cognitive performance data and meta-analytic techniques to investigate the relationships between THC-related biomarkers, subjective intoxication, and impairment.
Design and Methods of the Study
The researchers examined the relationship between subjective ratings of intoxication and impairment as “individuals using cannabis should self-evaluate their fitness to drive regardless of the regulatory approach being applied”.
Relevant studies were identified using online databases Web of Science and Scopus. All studies relating to driving impairment and THC (including specific medications such as Nabiximols, Sativex, dronabinol, Marinol, and namisol).
Studies also had to have been eligible for inclusion in the quantitative synthesis. Moreover, THC must have been administered via inhalation or oral ingestion. They also must have measured performance on an eligible domain. Finally, they must measure and report subjective intoxication and/or one or more THC-related biomarkers at an appropriate time relative to the performance test(s).
The researchers used several driving-related performance domains.
Four-level restricted maximum likelihood meta-regression analyses were performed to investigate the relationships between different THC-related biomarkers, subjective intoxication, and impairment of driving and driving-related cognitive skills in regular and ‘other’ (mostly occasional) cannabis users.
A total of 28 publications were included in the current review. These covered 822 eligible outcomes assessing the effects of THC on either subjective intoxication and/or THC related-biomarkers.
Results of the Study
The researchers note that, while per se concentrations of THC and THC-metabolites are often used to identify cannabis-impaired drivers on public roads and in the workplace, “research validating the relationships between THC-related biomarkers, subjective intoxication, and impairment of driving and driving-related cognitive skills is relatively limited.”
The analyses of existing evidence found that there were significant, linear relationships between most THC-related biomarkers and impairment of driving and driving-related cognitive skills in occasional cannabis users. However, each analysis (except one) contained a significant amount of residual heterogeneity, suggesting that these biomarkers are not a reliable measure to predict impairment.
Most of the ‘biomarker-performance’ relationships were found to be weak in strength. The use of blood samples to detect THC and THC-metabolites was found to be the poorest measure of impairment, demonstrating a ‘very weak’ relationship after both ingestion and inhalation. These results suggest that cannabis users who are not necessarily impaired may be charged with dangerous driving/driving under the influence, while some cannabis-impaired drivers may be mistakenly identified as not-meaningfully-impaired when per se limits are used to determine impairment.
The results from existing studies indicate that blood THC, 11−OH-THC and 11−COOH-THC concentrations, oral fluid THC concentrations, and subjective ratings of intoxication are relatively poor indicators of cannabis-induced impairment.
The authors conclude that the use of a per se method (which is currently used in seven US states) to identify cannabis-impaired drivers should be reconsidered. This approach can carry significant risks of unimpaired drivers being mistakenly identified as ‘cannabis-impaired’ (and vice versa).