A new study from Arizona State University in the US has uncovered another hidden health hazard in illicit cannabis.  

When investigating cannabis seized in Arizona and California the researchers found that 16% of cannabis samples were contaminated with the soil fungi Fusarium — fungal toxins that exceed safety limits for agricultural products.

After further research, the team declared that with no obvious signs of contamination in the final product, some cannabis users who have a range of other health issues could be exposing themselves to harmful substances without realising the danger to their health.

The research is published in Environmental Health Perspectives.

“Fungal contaminants are a category of cannabis that has received a lot of attention in the media but is understudied,” says Dr Maxwell Leung – an assistant professor of mathematical and natural sciences at Arizona State University, and a member of the Medical Marijuana Testing Advisory Council of the Arizona Department of Health Services.

“Since most of the cannabis market in the U.S. is illegal, we aimed to investigate the scale of Aspergillus and Fusarium contamination in illegal cannabis”.

The research team analysed cannabis samples collected between November 2023 and June 2024 from law enforcement seizures. These samples were tested for 23 different types of mycotoxins and fungal metabolites. Notably, Fusarium mycotoxins were found at levels that could pose serious risks to consumers.

“The presence of Fusariummycotoxins indicates the presence of Fusariumfungi in illicit cannabis, which can result in opportunistic infections in medical cannabis users who are immunocompromised due to health conditions such as cancer and HIV infection,” Leung says.

“Many Fusariummycotoxins can cause vomiting. This resembles cannabis hyperemesis which is linked to the increase in emergency room visits.”

Illegal cannabis is more susceptible to dangerous fungi contamination due to growing in unregulated environments with poor handling, unsanitary conditions, and a lack of oversight. 

The illicit cannabis market remains a dominant force in the United States. In 2022, it was estimated to be worth $76 billion, more than double the size of the legal market. A 2024 report from the U.S. Department of Cannabis Control noted that the unlicensed market consistently undercuts its licensed counterparts in terms of price. And, with tax hikes likely to be applied across states in the coming months, more users may be driven to the illegal market.

The toxin problems are compounded by the fact that, while California physicians can recommend cannabis for conditions including arthritis, glaucoma, migraines, and seizures, most health insurance plans do not cover medical marijuana due to its federal legal status.

Leung’s team calls for urgent and rigorous monitoring of cannabis products. He notes that while it’s known Fusarium fungi can produce harmful mycotoxins, less is understood about how these toxins behave under heat—particularly the high temperatures involved in cannabis consumption methods like vaping.

“A key gap in the current research is the thermal stability of mycotoxins, specifically how much mycotoxins can survive the vaping or heating process,” Leung says. “We also need to understand the scale of Fusarium contamination in cannabis production in the U.S.”

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