Science & Health
Government-Funded Study Finds Marijuana Users Are Being Unfairly Jailed Over DUI Allegations
Published
on January 20, 2026
Marijuana DUI Laws In Nearly 20 States Are Criminalizing Sober Drivers, New Government-Funded Study Finds
Laws designed to prevent marijuana-impaired driving in nearly 20 states may be unintentionally criminalizing—and even jailing—drivers who show no signs of impairment, according to a new government-funded scientific study.
For years, lawmakers and regulators seeking to curb drug-impaired driving have relied on an approach modeled after alcohol enforcement: setting a numerical threshold for THC in the bloodstream beyond which a driver is presumed impaired. These so-called “per se” THC limits treat the presence of a certain amount of the cannabis compound as automatic evidence of intoxication.
But the new research suggests that this strategy is fundamentally misaligned with how cannabis affects the body—and may be ensnaring sober individuals who are no longer impaired.
“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 found.
The research was published in the peer-reviewed journal Clinical Chemistry and was partially funded by the National Institutes of Health and the State of California.
THC Levels Don’t Track Impairment The Way Alcohol Does
The findings add to a growing body of evidence showing weak or inconsistent links between THC blood levels and driving impairment. While large epidemiological studies indicate that recent marijuana use may slightly increase crash risk, researchers consistently find the effect is far smaller—and far less predictable—than that associated with alcohol.
“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 authors wrote.
Unlike alcohol, which is typically undetectable in the bloodstream within one to two days of use, THC is fat-soluble. Because of that, traces of the compound can remain detectable for days or even weeks after consumption—long after any psychoactive effects have worn off.
“Alcohol generally cannot be detected 1–2 days after last ingestion, whereas THC can be measured up to 30 days after last use,” the researchers noted.
Study Shows Sober Users Can Still Exceed Legal Limits
To generate the data, researchers examined 190 heavy cannabis consumers who were instructed to abstain from marijuana for 48 hours. Blood samples were taken both before and after controlled cannabis use to establish baseline THC levels, and participants were evaluated using a driving simulator.
Even after the abstinence period, many participants still registered THC concentrations that would exceed legal limits in states with per se or zero-tolerance laws—despite showing no measurable impairment.
“Current cannabis blood concentrations used to identify impaired drivers could land innocent people in jail,” the Association for Diagnostics & Laboratory Medicine said in a press release accompanying the study.
“Cannabis blood limit laws lack scientific credibility and are not an accurate determinant of when drivers should face criminal charges or not.”
Nearly 20 States Still Use Per Se Or Zero-Tolerance THC Laws
Despite the growing scientific consensus, per se and zero-tolerance marijuana DUI laws remain widespread.
According to the study:
- Six states use per se THC limits—commonly set at 2 or 5 nanograms per milliliter of blood.
- Twelve states enforce zero-tolerance laws, meaning any detectable amount of THC can be grounds for a DUI charge.
The authors 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.”
They cautioned that toxicology testing alone is insufficient, recommending instead a combination of field observations, behavioral assessments, and biological testing when evaluating suspected impairment.
“At present, the best protocol is a combination of observations in the field and toxicology testing,” the researchers wrote.
Broader Evidence Challenges Assumptions About Marijuana And Driving
The findings align with other recent research challenging common assumptions about cannabis and traffic safety.
A separate study published last year by researchers at Nationwide Children’s Hospital and Ohio State University found no evidence that marijuana legalization increased tolerant attitudes or behaviors toward driving after cannabis use, based on national traffic safety survey data.
Nonetheless, federal officials continue to emphasize marijuana-impaired driving risks. Earlier this year, President Donald Trump’s confirmed nominee to lead a key federal traffic safety agency said he plans to “double down” on public awareness campaigns about cannabis-impaired driving in coordination with the White House.
As the new study makes clear, however, researchers warn that current marijuana DUI laws may be punishing biology rather than behavior—and that reform may be necessary to prevent the continued criminalization of unimpaired drivers..
