| Title: | Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence |
| Authors: | Robert L Fitzgerald, Anya Umlauf, Raymond T Suhandynata, David J Grelotti, Marilyn A Huestis, Kyle F Mastropietro, Igor Grant, Thomas D Marcotte |
| Year: | 2025 |
| Journal: | Clinical Chemistry |
A new study from Robert Fitzgerald, Ph.D., Director of the CMCR Bioanalytical Laboratory, Tom Marcotte, Ph.D., Co-Director of the CMCR, and other colleagues at UC San Diego examined whether current per se cannabis driving laws accurately indicate impairment. Many U.S. states treat specific blood THC levels—such as 2 or 5 ng/mL—as automatic evidence of driving under the influence. However, THC behaves very differently from alcohol: it can remain detectable in blood for days or even weeks after last use.
In this study of 190 regular cannabis users who abstained for at least 48 hours, nearly half still exceeded zero-tolerance thresholds, and one-quarter remained above the 2 ng/mL per se limit. Importantly, driving-simulation performance did not differ between participants above these cutpoints and those below them. Even after smoking cannabis, THC levels rose only slightly by six hours, and many returned to baseline.
The findings highlight a key mismatch between biology and policy. Regular users may test “impaired” under per se laws despite showing no measurable driving deficits, raising concerns about fairness and scientific validity. As cannabis legalization expands, evidence-based approaches to identifying genuinely impaired drivers are urgently needed.
Read the full publication here.
Citation: Robert L Fitzgerald, Anya Umlauf, Raymond T Suhandynata, David J Grelotti, Marilyn A Huestis, Kyle F Mastropietro, Igor Grant, Thomas D Marcotte, Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence, Clinical Chemistry, 2025;, hvaf121, https://doi.org/10.1093/clinchem/hvaf121