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.
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