Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations...

Several US states have per se laws using 2 or 5 ng/mL of delta-9-tetrahydrocannabinol (THC) as cutpoints for driving under the influence of cannabis, while some have zero-tolerance statutes. These cutpoints are considered prima facia evidence of driving impairment.

In a cohort of people who regularly use cannabis (N = 190) we measured baseline concentrations of THC after instructing participants to abstain from cannabis for at least 48 hours. Baseline driving performance was evaluated using a driving simulator. We also measured blood THC concentrations serially following a smoking session (placebo or active cannabis).

Forty-three percent of the participants exceeded zero-tolerance statutes (≥0.5 ng/mL) at baseline. Twenty-four percent had baseline blood THC concentrations that were ≥2 ng/mL and 5.3% were ≥5 ng/mL. The maximum observed baseline blood concentration was 16.2 ng/mL. Six hours after smoking active cannabis, the median (interquartile range) difference in THC concentrations compared with baseline was 0.5 (0-0.9) ng/mL; a 1-sample t-test comparing the mean change to 0 was significant (P < 0.001). There was no difference when comparing the mean change to 0 in the placebo group (P = 0.69). Simulated driving performance was not different between those who exceed zero tolerance and per se cutpoints vs those who are below these cutpoints (P > 0.05).

Many regular users of cannabis exceed zero tolerance and per se THC cutpoint concentrations days after their last use, risking legal consequences despite no evidence of impairment.