(Press-News.org) Scientists from the Center for Medicinal Cannabis Research (CMCR) at the University of California San Diego School of Medicine found that, in the largest such study to date, frequent cannabis users did not display impairments in driving performance after at least 48 hours of abstinence. The new findings have implications for public health as well as the enforcement of laws related to cannabis and driving.
Approximately three-quarters of Americans live in a state where cannabis is legally available, and about 15% of Americans currently use cannabis. As cannabis usage becomes more widespread, understanding its effects on daily activities like driving is crucial to maintaining public safety and appropriate legislation around cannabis use.
While acute cannabis intoxication can impair driving, it can be challenging to enforce cannabis and driving laws because, unlike alcohol, there is no biological test (e.g., blood concentrations) that directly relates to cannabis intoxication. Tetrahydrocannabinol (THC), the psychoactive compound in cannabis, can be detected in blood for several days to weeks after use, and in urine for weeks or even months.
One unanswered question around cannabis and driving is whether frequent cannabis users, who may or may not still have THC detectable in their blood, experience reduced driving abilities when not actively high. To answer this question, researchers analyzed data from two studies. The first, a randomized clinical trial, assessed driving performance in a sample of 191 cannabis users, all of whom had abstained for at least 48 hours. The second study compared a subset of the most frequent users from the first study with a smaller comparison group of people who don’t use cannabis. A driving simulator was used to assess driving performance and potential impairment.
The researchers found no indications of reduced driving ability in cannabis users who had abstained for at least two days. First-author Kyle Mastropietro, a graduate student in the San Diego State University/UC San Diego Joint Doctoral Program in Clinical Psychology, commented, “We did not find any relationship between driving performance, and cannabis use history or time of abstinence, nor blood THC concentrations. Of note, the most intensive users from the group, who mostly used cannabis daily and smoked an average of four joints per day, did no worse during this period of abstinence than a healthy, non-using comparison group.”
Thomas Marcotte, Ph.D., professor of psychiatry at UC San Diego School of Medicine and the study’s senior author, added, “The findings add to the growing body of evidence that relying on blood THC concentrations in regular cannabis users as possible indicators of impairment is not justified, given that THC may be detectable many days (or longer) after use.” Marcotte, who is also the co-director of the CMCR, added that “the findings reinforce the challenges in relating findings from cognitive testing in very frequent users who are abstinent to how they might function during real-world, overlearned behaviors like driving.”
The authors note that this study was done in a controlled lab environment, did not address all possible driving scenarios, and the nonuser comparison group was small. That said, the findings do provide new insights into the potential risks and consequences of chronic cannabis use.
The study was published in Psychopharmacology on September 6, 2025. It was funded, in part, by the State of California award to the CMCR via Assembly Bill 266 (Bonta/Cooley/Jones-Sawyer/Lackey: Agreement #907). The authors declare no conflicts of interest.
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Research alert: Frequent cannabis users show no driving impairment after two-day break
2025-09-11
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