Fentanyl and Cannabis: What a Decade of U.S. Seizure Data Shows
A large analysis of U.S. drug-seizure data found that fentanyl rarely appeared alongside cannabis, challenging claims that marijuana is commonly contaminated with the powerful synthetic opioid.
The study published in The Lancet Regional Health—Americas examined 11,940,207 samples reported by forensic laboratories across all 50 states and Washington, D.C., between January 2013 and December 2023. Researchers from Harvard Medical School, Brown University and the U.S. Food and Drug Administration analyzed fentanyl’s co-occurrence with nine drug categories, including heroin, cocaine, methamphetamine and cannabis.
The study was partly supported by an FDA grant and used data from the National Forensic Laboratory Information System, a database of substances analyzed after law-enforcement seizures. It was not a federal study in the sense of being conducted solely by the government, and its findings describe tested seizures rather than every drug product sold or consumed in the United States.
Fentanyl was common in heroin but rare in cannabis
Fentanyl co-occurred with about half of heroin samples by 2023. Its presence was much less common in non-opioid drugs: roughly 2.7% of cocaine samples and 1% of methamphetamine samples contained fentanyl that year. Cannabis samples had one of the lowest rates, with fentanyl detected in about 0.4% nationally in 2023. Across the study period, cannabinoid co-occurrence remained below 0.3% in the researchers’ monthly analysis.
The authors concluded that the results showed “no evidence of widespread fentanyl co-occurrence with cannabis.” They also found that fentanyl’s presence in stimulant supplies was increasing in some regions, particularly parts of the Northeast. Those regional patterns matter: national averages can conceal elevated risks in specific states or local drug markets.
The results do not prove that fentanyl can never be found in a cannabis-related product. The researchers noted that their data cannot determine whether co-occurrence resulted from deliberate mixing, accidental contamination or other points in the supply chain. They also cautioned that seized drugs may not represent the products people ultimately buy, and that laboratory reporting practices vary.
Why the distinction matters
Fentanyl remains a major driver of fatal overdoses. The Centers for Disease Control and Prevention’s fentanyl guidance says illegally manufactured fentanyl is commonly sold as powder or pressed into counterfeit pills and may be mixed with drugs such as heroin, cocaine and methamphetamine. The new study supports those concerns while showing that cannabis is not a comparable national source of fentanyl exposure in the available seizure data.
This distinction became politically salient in 2024, when then-Sen. JD Vance cited law-enforcement anecdotes while warning that marijuana and candy-like products could contain fentanyl. Individual reports can warrant investigation, but anecdotes cannot establish how common a phenomenon is. The national evidence reviewed in the study does not support describing fentanyl-laced cannabis as widespread.
There have been isolated reports involving unregulated THC products and alleged opioid contamination. For example, Massachusetts officials announced charges in 2022 after an investigation involving black-market THC products that authorities said contained traces of fentanyl. Such cases underscore the need to test suspicious products, but they do not by themselves show that ordinary cannabis is broadly being laced with fentanyl.
Regulation does not eliminate every risk
Unregulated cannabis can still contain other contaminants or have inaccurate labeling. Legal cannabis systems generally require laboratory testing and product information that are absent from illicit markets. The New York State Department of Health’s cannabis-safety guidance says there have been no verified incidents of fentanyl-laced cannabis while warning that unregulated products are not guaranteed to be free of harmful ingredients.
Regulation therefore offers a more evidence-based response than treating an unverified claim as a national trend. Testing, accurate labeling and surveillance can help identify genuine contamination problems, while public-health messaging should distinguish cannabis risks from the well-documented dangers of fentanyl in heroin, counterfeit pills and some stimulant supplies.
The study’s central message is not that the illicit drug supply is safe. Fentanyl co-occurrence increased across the drug categories examined, and even relatively uncommon contamination can be deadly because many people exposed to fentanyl may have little or no opioid tolerance. Rather, the findings show why drug policy should be guided by measured prevalence and regional evidence—not by the most alarming anecdote.
