Dr. Tom Marcotte, PhD, Co-Director of the CMCR, and Robert Fitzgerald, PhD, Director of the CMCR Analytical Laboratory, are helping reshape how cannabis-related driving impairment is understood and evaluated. In a recent study published in Clinical Chemistry, the investigators examined whether commonly used “per se” blood THC thresholds—modeled after alcohol limits—accurately reflect driving impairment among regular cannabis users.
Both investigators were recently interviewed for the Clinical Chemistry podcast.
As explained in the interview, per se laws establish a fixed blood THC concentration (such as 2 or 5 ng/mL) above which a driver can be considered legally impaired, regardless of actual driving performance. Using a high-fidelity driving simulator, Marcotte, Fitzgerald, and colleagues assessed the performance of nearly 200 regular cannabis users who abstained from cannabis for at least 48 hours. The study found that a substantial proportion of participants still exceeded per se THC thresholds after abstinence, yet performed no worse on driving tasks than those below the thresholds.
“These findings raise important concerns about the scientific validity of per se cannabis laws,” Marcotte explained during the podcast interview. Participants with blood THC levels above legal cutpoints showed comparable driving performance to those below the cutpoints prior to any cannabis administration, suggesting that residual THC does not reliably indicate impairment.
Fitzgerald, who directs CMCR’s bioanalytical laboratory, highlighted how THC can persist in the bloodstream of frequent users long after acute effects have resolved. In some cases, individuals maintained baseline THC concentrations well above per se limits without evidence of functional impairment, underscoring the challenge of relying on blood levels alone.
Together, the researchers emphasize the need for more scientifically grounded approaches to identifying cannabis-impaired driving—ones that account for timing, route of administration, and individual differences, rather than fixed numerical thresholds. Their work adds to a growing body of evidence calling for policies that better align with the complex pharmacology of cannabis and real-world driving risk.
Listen to the podcast here.