Microbial Contamination and Mycotoxins in Cannabis Products

Quick answer

Cannabis flower is a plant material, and plants carry microbes: molds, yeasts and bacteria. Some molds make toxic compounds called mycotoxins. Testing programs check for both. For most healthy adults the documented risk is not well quantified, but for people with weakened immune systems, inhaled mold is a serious concern. This page is information only.

How evidence is labeled here

Plain labels such as promising but limited, insufficient evidence, evidence of harm and Proper evidence not found are explained on How We Label Evidence in Specialty Topics.

What the terms mean

• Microbial contamination means unwanted bacteria, yeast or mold on or in a product.
• Mycotoxins are toxic secondary metabolites that some fungi produce. [1]
• Yeast and mold counts are measures labs report against set limits. [2]

A 2023 review reports that the fungal community of cannabis and hemp flowers (the mycobiome) includes over 100 fungal species, among them plant pathogens and toxin producers. [1] The fungi of most concern it names are Aspergillus, Penicillium, Fusarium and Mucor. [1] The authors also said this fungal community has often been understudied in safety research. [1]

What testing checks

California's regulator lists microbial impurities and mycotoxins among the tests required on cannabis goods, as read on 2026-10-22, and says labs report each result on a certificate of analysis (COA) with pass or fail. [3] Laws change and this is not legal advice. A review of US methods says mycotoxin detection usually uses liquid or gas chromatography with tandem mass spectrometry, and immunoaffinity columns are used to isolate aflatoxins and ochratoxin A. [4] Allowed limits and methods differ between states. [4]

What studies found

Licensed vs illegal flower in Canada (2026, observational). Fifty legal and 50 seized illegal samples were tested. In legal products, 20% exceeded European Pharmacopoeia microbial limits. [2] Among illegal products, 55% exceeded the aerobic plate count thresholds and 73% surpassed yeast and mold limits. [2] Mycotoxins were not detected in legal products but were present in 12% of illegal samples. [2]

This is one study in one country. It does not predict what any product on a shelf holds, and it did not follow users for illness. Evidence level: observational laboratory findings.

Documented harm: inhaled mold and weakened immunity

The strongest signal is for people with compromised immune systems. A 1988 case report described a bone-marrow transplant recipient who smoked marijuana heavily for weeks before pulmonary aspergillosis. Cultures of the marijuana grew Aspergillus fumigatus with characteristics identical to the organism from his lung biopsy. [5] He died with disseminated disease. [5] A review in cancer care notes that microorganisms carried on cannabis leaves and flowers, when inhaled, could expose users, particularly immunocompromised patients, to opportunistic lung infections. [6] That paper tested ways to sterilize herb for such patients; we do not reproduce those methods.

Evidence level for this group: evidence of harm in case reports; frequency unknown. Proper evidence not found for reliable rates of infection among healthy adults who use cannabis; our 2026-10-22 PubMed searches returned laboratory studies and case reports rather than population risk estimates.

Who should check with a clinician first

People with cancer treatment, organ or bone-marrow transplants, HIV, or other immune-weakening conditions should talk with their care team before any cannabis product, especially anything inhaled. See Existing Conditions and Talking With a Health Professional. Seek urgent care for fever, worsening cough or trouble breathing.

Reading a report

A passing microbial result describes one batch on one test date. See How to Read a Cannabis Certificate of Analysis (COA) and Laboratory Variability, Batch Numbers and Test Dates. Related: Pesticides in Cannabis Products.