Largest Study Finds No Link Between THC Levels and Driving Performance in Experienced Cannabis Users


Combining Toxicology Testing and Field Sobriety Tests Could Enhance Identification of Cannabis-Impaired Drivers, Study Shows

Largest trial to date provides valuable insights into reducing the likelihood of false accusations in cases of driving under the influence of cannabis.

In light of increasing medical and recreational cannabis use, a recent study sheds light on a novel method to enhance the identification of cannabis-impaired drivers.

Researchers conducted a placebo-controlled, double-blinded study investigating the impact of cannabis on driving performance. The study, involving 191 participants, required subjects to smoke a cannabis cigarette containing either a placebo or delta-9-tetrahydrocannabinol (THC), the active ingredient in cannabis. Participants' blood, oral fluid (OF), and breath samples were collected and their driving performance was assessed over a five-hour period using a simulator. Additionally, law enforcement officers carried out field sobriety tests (FSTs) to determine impairment.

Interestingly, researchers found no direct correlation between THC levels in blood, OF, or breath, and driving performance. However, FSTs proved to be significant in distinguishing between participants who had consumed THC and those who had consumed a placebo, up to 188 minutes post-smoking.

Approximately 71 minutes after smoking, FSTs identified 81% of participants who had consumed active drug as impaired. Nonetheless, it was also found that 49% of placebo-consuming participants were incorrectly classified as impaired at this same time point. Remarkably, when a 2ng/mL THC cutoff in OF was combined with positive FST findings, the number of misclassified placebo participants dropped to zero, 86 minutes post-smoking.

The study thus concludes that requiring both positive toxicology results and FST observations significantly improves accuracy in identifying potential cases of driving under the influence of THC, particularly by decreasing false positives.

In the largest trial of its kind involving experienced cannabis users, the researchers found no relationship between THC levels in blood, OF, or breath, and driving performance. These findings uphold current practices in several U.S. regions that necessitate officer observations of impairment, complemented by toxicology testing, before prosecuting drivers suspected of being under the influence. The research further suggests that oral fluid testing may be more effective than blood testing in reducing false accusations of cannabis-impaired driving.

The study's authors highlight the importance of choosing an optimal THC cutoff level, and the need for a better understanding of how a full Drug Recognition Expert (DRE) exam compares with FSTs, to ensure road safety. Further research in these areas is expected to provide valuable insights into the accurate identification of cannabis-impaired drivers.

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