Is irradiation enough to purify cannabis? A Canadian study casts doubt on this.
Gamma irradiation is now widely used in the production of medicinal and recreational cannabis to reduce the microbial load of dried flowers. This method, intended to sterilize products before they are placed on the market , is nevertheless called into question by a study from McGill University , published in late 2025 in the scientific journal Toxins .
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Researchers have shown that while irradiation significantly reduces detectable bacteria and mold, it does not completely eliminate certain toxigenic fungi , nor the mycotoxins they produce. This situation is considered concerning, particularly for people with weakened immune systems.
Underestimated health risks
According to the research team, viable spores belonging to well-known genera such as Aspergillus , Penicillium , and Fusarium persist after irradiation. These fungi are capable of producing dangerous toxins, such as aflatoxins , ochratoxins , and deoxynivalenol (DON) .
However, nearly 70% of cannabis is consumed by inhalation , allowing these spores and residues to penetrate directly into the lungs. The study notes that cases of pulmonary mycoses have been documented in both immunocompromised patients and healthy users.
"A single viable spore is enough to cause disease; therefore, we had to push the analysis beyond the limits of the ELISA test," emphasizes Saji George , co-author of the study.
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Insufficient control methods
To assess contamination, researchers analyzed non-irradiated, irradiated and licensed flowers , combining three approaches: microbiological cultures, PCR/qPCR tests to detect fungal DNA, and ELISA analyses to quantify toxins.
The result: even when products meet Canadian regulatory limits for yeast and mold , traces of fungal DNA and residual mycotoxins remain detectable. The authors believe that the tests currently used in the industry may underestimate the actual risks because they do not consistently identify viable spores.
"Currently used, advanced analytical methods [...] do not allow for the detection of live spores," warns the McGill team.
Prevention and hygiene : key levers
Faced with these limitations, researchers emphasize the importance of prevention , rather than simply sterilizing at the end of the production chain. Cannabis flowers, rich in resins, provide a particularly favorable environment for fungal growth.
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“Cannabis flowers contain sticky resins that greatly facilitate contamination,” explains Mamta Rani , co-author of the study. She emphasizes the need for increased vigilance at all stages, from cultivation to storage.
Some companies, however, manage to produce flowers that are virtually free of contamination thanks to controlled environments and strict hygiene protocols . The McGill team is also collaborating with industry to explore biological solutions, such as the use of beneficial bacteria to limit fungal growth.
The authors do not advocate for the rejection of irradiation, but rather for an evolution of safety standards , particularly for medical cannabis . They call for controls combining cultural, molecular, and immunological analyses to better protect the most vulnerable consumers.
“We are not seeking to tarnish the image of the industry, but rather to ensure its viability and to provide guidelines for the production of safe products,” concludes Saji George .
Posted Jan 31, 2026, 1:23 PM UTC