Past Cannabis Use Found to Not Effect Driving Performance

Consumers who abstain from cannabis for 48 hours drive no differently than abstainers, according to driving simulator data published in the journal Psychopharmacology.

Researchers affiliated with the University of California at San Diego evaluated driving performance in a cohort of 191 cannabis consumers. Participants completed a 25-minute simulated drive following 48 hours of cannabis abstinence. In phase II of the study, a subset of near-daily consumers was matched against non-using controls.

“The current study showed no evidence of a dose-effect relationship between simulated driving performance following a brief abstinence period,” investigators reported. “The current study also shows no evidence of short-term residual effects on simulated driving performance when comparing frequent cannabis users to a healthy non-using comparison group.”

Despite subjects’ lack of psychomotor impairment, researchers acknowledged that some participants tested positive for THC at levels that would classify them as “under the influence” in states with either zero-tolerant or per se THC blood limits. That’s because THC and its metabolites remain present in bodily fluids for extended periods of time following cannabis abstinence, whereas the acute effects of cannabis on psychomotor skills largely dissipate within a few hours.

The study’s authors concluded: “In regular cannabis users who abstained for ≥ 48 hours, we found no evidence of residual cannabis effects on simulated driving performance. This included no relationship between driving simulator performance and cannabis use intensity, days of abstinence, or cannabinoid concentrations, nor differences on these measures when comparing the most frequent cannabis users to a non-using comparison group. … Results of this study have implications for how future policy might weigh different pieces of evidence in the absence of objective confirmation of acute cannabis intoxication, such as one’s cannabis use history or residual blood THC, in everyday determinations of impaired driving.”

Numerous studies have previously reported no correlation between the detection of either THC or its metabolites in blood, urine, saliva, and breath and impaired driving performance, stated NORML. Nevertheless, several states have enacted laws criminalizing drivers who operate a motor vehicle with trace levels of THC or THC metabolites, regardless of whether the driver is impaired.