Cannabis legislation is undergoing rapid transformation worldwide. More than fifty countries now permit the prescription of medicinal cannabis, while nine countries have legalised recreational use, granting over five hundred million people worldwide legal access to cannabis products.

As with staple crops and other commercially available herbal products, cannabis must be carefully regulated to protect consumer health. One of the most significant risks is microbial contamination, particularly the presence of fungal species and their associated mycotoxins, which can cause a range of adverse health effects and pose serious risks to immunocompromised populations.

A recent study published in the Open Access journal Toxins assesses the presence of fungi and mycotoxins in dried cannabis buds following gamma irradiation. The authors provide evidence that while gamma irradiation reduces harmful fungi, it does not fully eliminate either the organisms or their metabolites.

Gamma irradiation is widely used in the cannabis industry because it is considered more effective and less damaging than alternatives such as heat treatment or pasteurisation, allowing microbial control without major impacts on product quality. However, the study’s findings indicate that current sterilisation practices may underestimate contamination risks. As co-author Dr. Saji George notes:

“We are not trying to tarnish the industry, but to help make it more sustainable and provide guidelines for safer products. We need stricter safety standards, especially for medicinal cannabis.”

Mycotoxigenic fungi in cannabis

Mycotoxigenic fungi are moulds that produce toxic compounds known as mycotoxins. They commonly grow on crops such as cereals, nuts and spices, thriving in warm and humid environments. The dense, nutrient rich microenvironment surrounding cannabis flowers makes them particularly susceptible to colonisation by mycotoxin-producing fungi.

Like other plants, cannabis also harbours fungal endophytes, fungal species that live within internal plant tissues. These fungi typically persist asymptomatically, often playing a critical role in plant survival. They can enhance plant growth, provide resistance against pathogens and pests, and increase resilience during drought.

However, their behaviour may turn pathogenic under conditions of plant stress or environmental change. Once activated, certain endophytic fungi can initiate or enhance mycotoxin biosynthesis. Furthermore, inadequate post-harvest drying and storage can further exacerbate mycotoxin accumulation, increasing health risks for consumers.

Routine testing to protect public health

Fungal and bacterial contamination has previously been reported in more than 20% of cannabis intended for medicinal use. This contamination poses significant health risks, particularly for individuals with compromised immune systems, including cancer patients, transplant recipients, and people living with HIV/AIDS, populations for whom medicinal cannabis use is frequently prescribed.

Although no deaths have been directly attributed to cannabis toxicity itself, its use has been indirectly associated with fatal outcomes resulting from conditions such as pulmonary aspergillosis and other opportunistic infections arising from the consumption of contaminated cannabis. Notably, such infections have also been reported in otherwise healthy users.

As approximately 70% of cannabis is consumed via smoking or vaporisation, fungal spores and mycotoxins may be delivered directly to highly absorptive lung tissue, increasing the potential for harm.

Together these findings underscore the need for routine mycotoxin monitoring as part of standard safety protocols. While the Cannabis Safety Institute recommends routine testing for mycotoxigenic fungi, fewer than 0.5% of published cannabis studies address mycotoxins or spoilage organisms. This disparity highlights a major gap of knowledge in cannabis safety research, one that the present study aims to address.

A multi-method approach to uncover underestimated contamination risks

The authors analysed ten different dried cannabis bud samples obtained from a licensed producer operating within the legal market, alongside irradiated and non-irradiated controls for comparison. To capture different dimensions of contamination, the study employed three complementary analytical approaches:

• Enzyme-linked immunosorbent assays (ELISA) were used to quantify the presence of mycotoxins, providing a measure of residual toxic compounds that may still exist after irradiation.
• Polymerase chain reaction (PCR) assays targeted fungal DNA and genes associated with mycotoxin production, allowing detection of toxigenic fungi even when viable organisms were no longer present.
• Culture based methods were used to assess the survival of living fungi and bacteria following irradiation by promoting microbial growth under laboratory conditions.

ELISA is the current industry standard for detecting mycotoxin contamination, offering rapid turnaround times that are well suited to commercial supply chains. However, it detects only a limited range of toxins and may fail to identify trace mycotoxin levels following irradiation due to its limit of detection. Importantly, the absence of detectable mycotoxins does not equate to the absence of disease risk, as viable pathogens or fungal DNA encoding mycotoxin production may still persist.

To overcome this limitation, the researchers combined ELISA with PCR and culture-based methods, enabling detection of fungal DNA and viable organisms that would not be identifiable using ELISA alone. Emphasising the value of this integrated approach, co-author Dr Saji George notes:

“A single spore can cause disease, so we had to go beyond the ELISA limit to see. To the general population, this may not have much significance, but immunocompromised people will be at much higher risk.”

Regulatory compliance does not guarantee safety

Overall, the results showed that gamma irradiation substantially reduced fungal and bacterial loads in cannabis buds. All irradiated samples met Health Canada’s regulatory thresholds for culture-based microbial testing. No bacterial growth was detected following irradiation. However, fungal colonies were still recovered from treated samples, including several known mycotoxigenic and pathogenic species.

Notably, Aspergillus spores were detected in irradiated samples. These spores have previously been reported to exhibit resistance to gamma radiation, highlighting a critical limitation of post-harvest sterilisation. Once contamination is established, complete removal becomes extremely difficult, underscoring the importance of prevention. As co-author Dr Mamta Rani explains:

“Cannabis buds have sticky resins, so they are really susceptible to contamination. These fungi are everywhere, so we need to be more careful at every stage, from growing and harvesting to processing and storage.”

Molecular evidence of persistent contamination

PCR analysis further confirmed the presence of biosynthetic genes associated with mycotoxin production across all sample types. While the presence of these genes does not directly indicate active toxin production, the consistent detection across samples raises safety concerns.

Importantly, gene copy number analysis revealed no meaningful difference between irradiated and non-irradiated samples, indicating that gamma irradiation did not fully degrade fungal genomes.

Residual mycotoxins detected after irradiation

ELISA analysis detected major mycotoxins in all tested samples. Notably, cannabis products obtained from licensed producers tested positive for one or more mycotoxins, demonstrating that regulatory compliance does not preclude the presence of toxic residues.

The results demonstrate that while gamma irradiation can reduce microbial loads below regulatory thresholds, it does not reliably eliminate mycotoxigenic fungi, fungal DNA, or residual mycotoxins. As a result, current regulatory frameworks may still permit products that pose credible health risks, particularly for immunocompromised medicinal cannabis users.

Rethinking cannabis safety

The results from this study indicate that gamma irradiation alone is insufficient to ensure the removal of mycotoxins from cannabis buds. Consequently, contamination must be addressed proactively across the whole supply chain, rather than relying solely on end-point sterilisation.

The authors propose preventative strategies, including:

• Cultivating cannabis under conditions that suppress toxigenic fungal growth.
• Implementing rigorous post-harvest handling, drying and storage practices.

Achieving such rigorous procedures is possible, as Dr. Mamti Rani states:

“It is possible to produce clean cannabis. Some companies we work with have achieved this through strict hygiene practices and controlled environments,”

Increased cannabis legalisation enables stronger safety regulation

A key limitation of this study is its small sample size, with only ten different licensed cannabis products analysed. However, this limitation likely reflects the regulatory and logistical challenges associated with accessing cannabis for research. While the contaminants found may not be consistent across regions or cannabis producers, they nonetheless raise important concerns that warrant further investigation.

Despite these constraints, the study highlights the value of collaboration between policymakers and researchers in identifying emerging safety concerns in publicly available cannabis products. More permissive regulations can support both industry and research efforts to develop rigorous safety protocols, which can inform policymakers and help ensure public safety. However, such regulatory shifts remain controversial due to concerns surrounding public health and misuse. As legal frameworks expand globally, increased access to research materials should facilitate more comprehensive investigations into contamination risks. Open Access publishing, such as that supported by MDPI, plays an important role in ensuring that these findings are readily available to both researchers and policymakers alike.

Ultimately, improving cannabis safety will require a shift in regulatory focus, from simply meeting numerical contamination thresholds to evaluating the type, persistence, and toxic potential of microbial contaminants, particularly those capable of producing mycotoxins.

More studies on microbial toxins and medicinal cannabis use can be found across the Open Access journals International Journal of Molecular Sciences, Psychoactives, and Toxins. Alternatively, you can access the full MDPI journal list here.