Study Examines Multiple Methods of Testing Driving Impairment after Consuming Cannabis

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A recent study reported results from a University of California San Diego Center for Medicinal Cannabis Research study on cannabis consumption and its effects on driving ability.

A study published May 25, 2023 in the journal Clinical Chemistry evaluated the effects of tetrahydrocannabinol (THC) on driving performance (1). The placebo-controlled randomized double-blinded protocol studied 191 participants who were instructed to smoke as much or as little of a THC cigarette that was either a placebo or contained delta-9-tetrahydrocannabinol (1).

“The relationship between blood THC concentrations and crash risk is not established, but there is a clear understanding that THC impairs driving performance in many, but not necessarily all, individuals,” the introduction stated (1). “The question that remains is how to best identify drivers who are impaired by cannabis.”


After smoking, the participants gave blood, oral fluids (OF), and breath samples. Law enforcement officers from the California Highway Patrol or other agencies performed the sobriety tests on the participants and classified them as impaired or not impaired.

“In the largest trial to date involving experienced users smoking cannabis, there was no correlation between THC (and related metabolites/cannabinoids) in blood, OF, or breath and driving performance,” the researchers concluded (1). “Our data support the current practice in many areas of the United States that requires officer observations of impairment along with toxicology testing before prosecuting drivers for being under the influence.”

The researchers suggested evidence for using oral fluids as opposed to blood samples in reducing the likelihood of false accusations of impairment, and stated that more research is needed on the comparison between exams from drug recognitions experts and field sobriety tests (1).


  1. Fitzgerald, R. L.; Umlauf, A.; Hubbard, J. A.; Hoffman, M. A.; Sobolesky, P. M.; Ellis, S. E.; Grelotti, D. J.; Suhandynata, R. T.; Huestis, M. A.; Grant, I.; Marcotte, T. D. Driving under the Influence of Cannabis: Impact of Combining Toxicology Testing with Field Sobriety Tests. Clinical Chemistry 2023.