Mycotoxins in Cannabis: What Researchers Found, and What Remains Unknown

Fungal contamination in cannabis is a legitimate quality and safety concern, but the available evidence is more nuanced than the phrase “moldy cannabis” may suggest. Some fungi found on cannabis can produce mycotoxins—naturally occurring compounds associated with serious health effects—yet detecting a potentially toxigenic fungus does not by itself prove that toxins are present in a product.

Mycotoxins are secondary metabolites produced by certain molds. The best-known examples include aflatoxins and ochratoxin A. Aflatoxin B1 is considered the most potent of the major aflatoxins and is associated with liver toxicity and cancer risk. Ochratoxin A has been linked primarily to kidney damage and may also affect the immune system. The World Health Organization’s overview of mycotoxins notes that these compounds can develop when crops are exposed to warm, humid conditions and that many are sufficiently stable to survive processing.

What the cannabis study actually found

The research most closely associated with this topic is the 2015 study “Cannabis microbiome sequencing reveals several mycotoxic fungi native to dispensary grade Cannabis flowers”. Researchers examined 17 dispensary-derived cannabis samples obtained from Massachusetts and Amsterdam. The samples were collected in different years and analyzed using fungal DNA sequencing alongside culture-based testing.

Six of the 17 samples tested positive for yeast and mold DNA in the study’s quantitative PCR screen. Sequencing identified several fungal species, including potentially toxigenic members of the Aspergillus and Penicillium genera. Some organisms detected by sequencing were not recovered by conventional culture methods, highlighting a limitation of relying on a single testing approach.

However, the study did not demonstrate that every detected fungus was actively producing mycotoxins, nor did it establish the amount of toxin a consumer might inhale or ingest. Fungal DNA can remain in a sample even when organisms are no longer viable, and the ability of a species to produce a toxin depends on factors such as strain, moisture, temperature, nutrients and storage conditions.

Why moisture and handling matter

Mold growth becomes more likely when harvested cannabis remains damp or is stored in conditions with high humidity and poor air circulation. Rain during harvest, inadequate drying, condensation, and improper packaging can all increase the risk of fungal growth during cultivation, transport or storage.

Environmental conditions alone do not determine whether mycotoxins will form. The relevant fungal species must be present, and the conditions must support both growth and toxin production. For that reason, a total yeast-and-mold count is not the same as a measurement of mycotoxin contamination. Many environmental fungi are not known to produce toxins, while some potentially important organisms may be missed by broad culture-based tests.

Exposure risks are not yet well defined

Mycotoxins have been studied extensively in food and animal feed, but much less is known about exposure from inhaling contaminated cannabis smoke or vapor. Research on dietary aflatoxin and ochratoxin exposure can help identify possible hazards, but it cannot establish the risk associated with every cannabis product or method of consumption.

This distinction is particularly important for medical cannabis users who may be immunocompromised or have underlying respiratory disease. Inhaling fungal material can pose risks independent of mycotoxins, including allergic reactions and, in vulnerable people, opportunistic fungal infections. The presence of a fungus in a laboratory sample does not mean that illness will occur, but it is one reason microbial quality controls are important.

Testing requires more than a single screening method

Laboratories may use culture, polymerase chain reaction, immunoassays and chromatography to investigate fungal contamination and mycotoxins. Enzyme-linked immunosorbent assays, or ELISAs, can provide relatively rapid screening for selected toxins. More specific confirmation is generally performed with liquid chromatography coupled to mass spectrometry, although these methods require trained personnel, validated procedures and appropriate reference standards.

Researchers have also demonstrated that culture-independent sequencing can reveal organisms missed by traditional plating. A later review of fungal and mycotoxin contamination in cannabis and hemp flowers emphasized that finding a toxigenic species is not equivalent to proving that it produced a toxin under the conditions present in the cannabis sample.

Testing methods must therefore be validated for the particular cannabis matrix being analyzed. Sampling is another challenge: contamination may be unevenly distributed within a batch, meaning a small or unrepresentative sample may not reflect the entire product.

Regulatory limits are not interchangeable

The U.S. Food and Drug Administration’s 20-parts-per-billion limit requires qualification. The FDA’s 20 ppb action level applies to total aflatoxins in specified human foods; it is not a universal federal limit for cannabis products. Cannabis testing requirements are established through state programs and differ in the contaminants covered, analytical methods used and allowable thresholds.

Some states have adopted explicit cannabis requirements for aflatoxins, ochratoxin A or other contaminants. Massachusetts, for example, publishes formal testing requirements for microbiological contaminants and mycotoxins. These rules can change, so producers and consumers should consult the regulator responsible for the jurisdiction in which a product is manufactured or sold.

The 2015 sequencing study was an important early demonstration that dispensary cannabis can harbor fungal communities that conventional testing may not fully capture. It was not, however, a prevalence survey of the entire cannabis market or proof that all samples containing potentially toxigenic fungi contained dangerous toxin concentrations.

More research is needed to determine which fungi produce mycotoxins in cannabis, how cultivation and storage conditions affect production, whether toxins persist through drying and processing, and what exposure levels may result from smoking or vaporization. Until those questions are answered, reliable sampling, validated analytical methods and clear, consistent regulatory standards remain the most important safeguards for cannabis quality.