A new study demonstrates that many regular marijuana users exceed legal blood cannabis limits for U.S. drivers despite showing no evidence of impairment.
The findings, published in Clinical Chemistry, contribute to evidence that cannabis blood limit laws lack scientific credibility and are not an accurate determinant of when drivers should face criminal charges or not.
Previous research that evaluated the effect of delta-9-tetrahydrocannabinol (THC, the main psychoactive compound in cannabis) on motor vehicle crashes concluded that there is no increase of crash risk because of detectable THC.
Legal limits and real-world implications
Despite evidence showing no correlation between the detection of THC in the blood and driving impairment, six states in the United States have per se laws using 2 or 5 ng/mL of THC as the cut-off point for driving under the influence of cannabis, while 12 have a zero-tolerance law.
These cut-off points are considered face value evidence of driving impairment, which means that even if it has been several days since an individual’s last use of the drug and they show no behavioral impairment, they may still face legal risks, up to and including felony charges.
To address this issue, a team of researchers led by Dr. Thomas D. Marcotte, professor of psychiatry at the University of California, San Diego, and codirector of the Center for Medicinal Cannabis Research, set out to investigate the blood concentrations of THC in regular cannabis users, as well as the simulated driving performance for participants who exceeded per se cut-off points compared with those who were below these values.
The researchers measured baseline concentrations of THC in 190 regular cannabis users after instructing participants to abstain from cannabis for at least 48 hours. Following abstention, the researchers also evaluated driving performance in this group using a driving simulator.
Study findings challenge current laws
From this, the team found that many regular users of cannabis exceed zero tolerance and per se THC cut-off point concentrations days after their last use. Specifically, 43% of participants exceeded zero-tolerance statutes at baseline, while 24% had baseline blood THC concentrations that were greater than or equal to the per se cut-off of 2 ng/mL, and 5.3% had blood concentrations greater than or equal to 5 ng/mL.
Based on the results from the driving simulation, participants with elevated baseline concentrations of THC did no worse on a driving simulator compared with participants who were below per se cut-off points. Altogether, the results add to a growing body of evidence showing that current per se THC blood limit laws lack scientific credibility as face-value evidence of impairment.
Calls for better impairment assessment
“More work needs to be done to address how to best identify drivers who are under the influence of cannabis and are unsafe to drive,” the study authors wrote. “At present, the best protocol is a combination of observations in the field and toxicology testing.”
They also added that “an essential component of improving highway safety is collaborations between law enforcement and the scientific community to develop standards that are unbiased and potentially lifesaving.”
More information:
Robert L Fitzgerald et al, Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence, Clinical Chemistry (2025). DOI: 10.1093/clinchem/hvaf121
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