A two-year study conducted by the Center for Medicinal Cannabis Research at UC San Diego School of Medicine found that at least 50% of people with THC in their bloodstream were "impaired" when undergoing driving simulations.
Researchers recruited 191 regular cannabis users to partake of cannabis containing different levels of delta-9-tetrahydrocannabinol -- the psychoactive compound in cannabis -- or a placebo immediately before a series of driving simulation tests over several hours.
According to the findings, published in Wednesday's online issue of JAMA Psychiatry, the THC group displayed significantly diminished ability on a composite drive score that assessed key simulated driving variables such as swerving in lane, responding to divided attention tasks and following a lead car. The non-placebo participants smoked a cannabis cigarette with either 5.9% or 13.4% THC.
Get top local stories in San Diego delivered to you every morning. Sign up for NBC San Diego's News Headlines newsletter.
However, not all displayed significantly diminished driving skills compared to the placebo group. The researchers said around 50% could be described as impaired.
"Our study of a large group of regular users underscores the complexity in understanding the relationship between cannabis intake and driving decrements, reinforces the challenges in communicating the varying level of risks associated with use and the difficulty in identifying the subset of individuals most at risk for impaired driving," said first and senior author Thomas Marcotte, co-director of CMCR and a professor of psychiatry at UCSD School of Medicine.
Sign up for our Breaking news newsletter to get the most urgent news stories in your inbox.
Local
The comparative driving skill decline was sharpest at the 30 minute and one-hour, 30-minute marks after inhaling cannabis, then leveled to borderline differences with the placebo at the three-hour, 30-minute mark, with no differences at four hours, 30 minutes.
The study authors said driving scores did not differ based on the THC content of the cigarette. Both the 5.9% and 13.4% groups performed similarly, suggesting that users "self-titrated" by smoking in such a way to achieve similar highness levels.
"Although users in the THC group felt impaired and were hesitant to drive at 30 minutes, by one hour, 30 minutes they believed the impairment was wearing off and were more willing to drive," Marcotte said. "This was despite their performance not significantly improving from the 30-minute point. This may indicate a false sense of safety, and these first few hours may constitute a period of greatest risk since users are self-evaluating whether it is safe to drive."
The study found no relationship between post-smoking blood THC concentrations and simulator performance.
Co-author Robert Fitzgerald, professor of clinical pathology at UCSD School of Medicine, director of the Toxicology Laboratory and associate director of Clinical Chemistry Laboratory at UCSD Health said, "The complete lack of correlation between blood concentrations and driving performance was somewhat surprising. It's strong evidence against developing 'per se' driving under the influence statutes."
Per se laws, Latin for "by itself," establish a statutory violation if a legal standard is breached, such as blood-alcohol concentration in driving under the influence laws.
The findings, said the authors, indicate that cannabis use resulted in diminished driving ability -- in simulators -- but when experienced marijuana users controlled their intake, impairment could not be inferred based on THC content of the cigarette, behavioral tolerance or THC blood concentrations.
They wrote that future research should address factors such as individual biologic differences, personal experience with cannabis and cannabis administration methods in relation to driving impairment.
