Laws aimed at preventing marijuana-impaired driving in almost 20 states are causing innocent people who show no signs of impairment to be criminalized and imprisoned for allegedly operating vehicles while under the influence, a new government-funded study shows.
Lawmakers and regulators aiming to reduce drug-impaired driving have long sought to apply a familiar strategy from alcohol enforcement: setting a numerical limit of THC in the bloodstream beyond which a driver is presumed to be impaired, commonly referred to as a “per se” amount.
But the new study suggests that approach may be badly misaligned with the science related to impairment from cannabis, the components and metabolites of which can remain in the body day or weeks after use—when impairment is no longer an issue.
“Many regular users of cannabis exceed zero tolerance and per se THC cutpoint concentrations days after their last use, risking legal consequences despite no evidence of impairment,” the study, which was published in the scientific journal Clinical Chemistry and partially funded by the National Institutes of Health and the State of California, found.
The findings echo earlier research showing weak or inconsistent links between THC blood levels and crash risk. Large epidemiological studies have found that while marijuana use may slightly increase collision risk, the effect is far smaller than that of alcohol use.
“One of the primary problems with using THC concentrations in per se legislation is that the pharmacokinetics of THC are much different from ethanol,” the researchers wrote.
The authors noted that alcohol generally cannot be detected 1 to 2 days after last ingestion, whereas THC can be measured up to 30 days after last use because of its lipophilic nature.
To generate the data, researchers studied 190 heavy consumers who were instructed to abstain for 48 hours. Following that several day period, the participants’ blood THC concentrations were measured both before and after cannabis consumption to establish baselines. They were also observed using a driving simulator.
“Current cannabis blood concentrations used to identify impaired drivers could land innocent people in jail,” the Association for Diagnostics & Laboratory Medicine, which publishes the journal that the study appeared in, said in a press release.
“Cannabis blood limit laws lack scientific credibility and are not an accurate determinant of when drivers should face criminal charges or not.”
The authors of the study concluded that “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.”
“Despite evidence showing no correlation between the detection of THC in the blood and driving impairment, 6 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,” the journal’s press release says.
The authors, affiliated with the University of California, San Diego and the Center for Medicinal Cannabis Research cautioned that additional research is needed, saying “at present, the best protocol is a combination of observations in the field and toxicology testing.”
A separate study last year found “no support that marijuana legalization increased tolerant behaviors and attitudes toward driving after marijuana use.” Authored by researchers at Nationwide Children’s Hospital and Ohio State University, the study used data from a national traffic safety survey.
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