Adopting recreational cannabis laws, beyond only medical cannabis laws, may help reduce the size of the illegal cannabis market in U.S. states, reports a new study at Columbia University Mailman School of Public Health. The study is among the first to comprehensively examine illegal cannabis market dynamics using law enforcement seizure data. The findings are published in the International Journal of Drug Policy.
"Until now, there was little research assessing whether recreational cannabis legalization helped disrupt illegal markets," said Nicole Fitzgerald, PhD, postdoctoral research fellow in the Columbia Mailman School Department of Epidemiology and first author. "Additionally, few studies used law enforcement cannabis seizures to examine the extent to which legal cannabis markets may have displaced illegal cannabis markets."
As of 2025, 40 U.S. states and Washington, DC have legalized medical cannabis, while 24 states and DC have legalized recreational cannabis, though cannabis remains a Schedule I substance under federal law.
Using data from the High Intensity Drug Trafficking Areas Program (HIDTA), the team analyzed cannabis seizures made by participating law enforcement agencies in all 50 states and Washington, DC between 2010 and 2023. This is the first study to use HIDTA data to examine the effects of cannabis legalization on cannabis seizures.
The data included 286,844 cannabis seizures across 686 state-year observations. Researchers linked these data with cannabis policy information from the RAND-USC Opioid Policy Tools and Information Center.
Results showed that states adopting recreational cannabis laws in addition to medical cannabis laws experienced a 45 percent relative reduction in average cannabis seizure counts, compared with states that had only medical cannabis laws. The decline appeared both immediately after recreational cannabis law adoption and one year later, even after controlling for state demographic and law enforcement factors and time trends.
"The decrease in seizures may reflect a reduction in illegal cannabis supply, as consumers shift toward regulated markets and some illegal suppliers exit the market," observed Fitzgerald, who is also supported by the NIDA T32 Substance Abuse Epidemiology Training Program at Columbia Mailman School (SAETP).
"Another likely explanation is changes in law enforcement priorities. In jurisdictions where cannabis is legal, agencies may devote fewer resources to cannabis enforcement and instead focus on other drugs, such as illicitly manufactured fentanyl."
Illegal cannabis markets have not disappeared entirely in states with recreational legalization, Fitzgerald notes, but the findings suggest that regulated markets may be displacing part of the illegal supply.
"Our study contributes to the growing body of literature examining the impact of cannabis policy implementation on the illegal cannabis market," said Silvia Martins, MD, PhD, professor of Epidemiology, co-director of the SAETP T32 training program, and senior author. "Future research should qualitatively consider the effect of the changing cannabis policies on law enforcement behaviors and drug enforcement priorities."
"We also recommend additional studies examine different cannabis policy exposures, including the density of cannabis dispensaries, as well as other drug seizure outcomes such as quantities seized and patterns observed. Overall, these results suggest that the adoption of recreational cannabis laws, beyond the adoption of only medical cannabis laws, may help to reduce the size of the illegal cannabis market in those states."
Co-authors are Joseph Palamar, New York University Grossman School of Medicine, and Kechna Cadet, Columbia Mailman School and Columbia Vagelos College of Physicians and Surgeons, and Esther Rowan, and Emilie Bruzelius, Columbia Mailman School.
The study was supported by the National Institute on Drug Abuse, U.S. National Institutes of Health.
The authors report no financial conflicts of interest.
Columbia University Mailman School of Public Health
