Nearly half of regular cannabis users still had detectable THC in their blood after at least 48 hours without using, and one in four exceeded the 2 ng/mL per se driving limit.
Policymakers; traffic safety researchers; defense attorneys; regular cannabis users in per se law states.
24% of regular users exceeded 2 ng/mL after 48+ hours of abstinence
What the researchers found
Among 190 regular cannabis users who abstained for at least 48 hours, 43% exceeded zero-tolerance THC limits (0.5+ ng/mL), 24% exceeded the 2 ng/mL per se limit, and 5.3% exceeded 5 ng/mL. The maximum observed baseline THC concentration was 16.2 ng/mL. Six hours after smoking, the median THC increase above baseline was only 0.5 ng/mL.
Why it matters
Several US states use THC blood concentration as legal proof of driving impairment, similar to blood alcohol limits. This study shows that regular users can exceed these limits days after their last use, when they are not impaired.
The numbers in context
190 regular cannabis users. After 48+ hours abstinence: 43% exceeded zero-tolerance (0.5+ ng/mL), 24% exceeded 2 ng/mL, 5.3% exceeded 5 ng/mL. Max baseline: 16.2 ng/mL. Post-smoking median increase above baseline: only 0.5 ng/mL at 6 hours.
How the study worked
Prospective cohort study of 190 regular cannabis users. Measured baseline blood THC after 48+ hours of abstinence, then measured serial THC levels after a controlled smoking session. Also assessed driving performance via driving simulator.
What this study cannot tell us
Self-reported 48-hour abstinence (though confirmed by declining THC levels). Driving simulator may not fully replicate real-world driving. Study did not assess actual driving incidents or crashes.
How to read the evidence
Well-designed prospective study with objective biological measures, though driving simulation rather than real-world assessment.
When this study was published
2025 study
The bigger picture
Per se THC driving laws were modeled on alcohol impairment laws, but THC pharmacokinetics are fundamentally different. THC accumulates in fat tissue and releases slowly, meaning blood levels don't correlate with impairment the way blood alcohol does.
Questions still open
- Should per se THC driving limits be reconsidered given these pharmacokinetic realities? What alternative measures could better assess cannabis-related driving impairment? How long would regular users need to abstain to reliably fall below legal limits?
Common questions
What are per se driving laws?
Why do regular users still have THC in their blood after days without using?
Read the original research
Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence.
Clinical chemistry, 71(12), 1225-1233
Citation
Fitzgerald, Robert L; Umlauf, Anya; Suhandynata, Raymond T; Grelotti, David J; Huestis, Marilyn A; Mastropietro, Kyle F; Grant, Igor; Marcotte, Thomas D. (2025). Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence.. Clinical chemistry, 71(12), 1225-1233. https://doi.org/10.1093/clinchem/hvaf121