Research from the Center for Medicinal Cannabis Research (CMCR) at the University of California San Diego School of Medicine reveals that frequent cannabis users do not exhibit impaired driving performance after a minimum of 48 hours of abstinence. This study, the largest of its kind to date, highlights significant implications for public health and the enforcement of cannabis-related driving laws.
As cannabis becomes increasingly legalized across the United States—approximately three-quarters of Americans reside in states where it is legally available—and with roughly 15% of the population currently using it, understanding its effects on activities such as driving is essential for public safety. While acute cannabis intoxication is known to impair driving, establishing legal thresholds for cannabis use is complicated. Unlike alcohol, there is no straightforward biological test that correlates with cannabis impairment, as the psychoactive compound tetrahydrocannabinol (THC) remains detectable in blood for days or even weeks after consumption.
Study Overview and Findings
To address concerns about the impact of cannabis on driving abilities, researchers analyzed data from two studies. The first involved a randomized clinical trial assessing driving performance in 191 cannabis users who 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 group of non-cannabis users. A driving simulator was employed to examine driving performance and identify potential impairment.
The findings were clear: there were no indications of diminished driving capabilities in cannabis users who had refrained from use for two days. First-author Kyle Mastropietro, a graduate student in the San Diego State University/UC San Diego Joint Doctoral Program in Clinical Psychology, emphasized, “We did not find any relationship between driving performance, cannabis use history or time of abstinence, nor blood THC concentrations.” Notably, the most intensive users, who typically consumed an average of four joints daily, did not perform worse than the non-using comparison group during the abstinence period.
Implications for Public Safety and Law Enforcement
The study’s senior author, Thomas Marcotte, Ph.D., a professor of psychiatry at UC San Diego School of Medicine, pointed out that the results contribute to the growing evidence suggesting that blood THC concentrations should not be relied upon as indicators of impairment in regular cannabis users. “THC may be detectable many days after use,” he noted, underscoring the complexities involved in relating cognitive testing results from frequent users to their real-world driving behaviors.
While the research provides valuable insights, the authors caution that the study was conducted in a controlled laboratory environment and did not cover all possible driving scenarios. Additionally, the small size of the non-user comparison group may limit the breadth of the findings. Nevertheless, the results offer new understanding of the potential risks associated with chronic cannabis use.
The study was published in the journal Psychopharmacology on September 6, 2025, and was partially funded by a State of California award to the CMCR through Assembly Bill 266 (Bonta/Cooley/Jones-Sawyer/Lackey: Agreement #907). The authors have declared no conflicts of interest, reinforcing the credibility of their findings in the ongoing discourse surrounding cannabis use and public safety.