James Duckenfield, CEO of Glass Pharms®, examines what irradiation changes in medical cannabis flower, what it leaves behind, and why understanding microbial quality requires a closer look at the science.
Much of the discussion involving irradiated medical cannabis centres on whether the treatment affects THC or terpenes. Those are reasonable questions, but I think the microbiology deserves an equal share of the attention.
Irradiation can substantially reduce the number of viable microorganisms in flower, but any mould that has already grown remains physically present after treatment.
Careful cultivation, hygiene, and post-harvest handling help prevent that growth from occurring in the first place. Testing allows producers to check that those controls are working, while drying and storage conditions determine whether surviving microorganisms have the opportunity to multiply once the batch of flower has been harvested. Irradiation needs to be understood within that wider process.
What does irradiation do, and what does it leave behind?
Ionising radiation damages microbial DNA and other cellular structures, preventing affected organisms from reproducing. Gamma treatment, electron-beam treatment, and X-ray treatment deliver that energy in different ways. None makes the flower radioactive.
The effectiveness of treatment depends on the dose, the organisms being targeted, and the conditions under which it is applied. Research on medical cannabis has demonstrated reductions of over 99.9% in viable fungal counts following irradiation, with electron-beam treatment achieving around 99.999% under the conditions tested. This sounds great until you appreciate that this does not mean sterilisation.
For flower initially containing ten million colony-forming units per gram, a 99.999% reduction would leave approximately 100 CFU/g. By that measure, the treatment is highly effective, but it does not make the flower sterile.
Crucially, these tests measure viable microorganisms capable of forming colonies under laboratory conditions, not the total amount of fungal material remaining in the flower. Hyphae, spores and fungal fragments may still be present, including material capable of provoking allergic reactions. And as the US Environmental Protection Agency explains, mould can remain allergenic even when dead.
A low viable count cannot, on its own, demonstrate that medical cannabis flower is free from the residues of earlier mould growth, including aflatoxins.
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What does the research show about treated flower?
THC and CBD appear relatively stable under the irradiation conditions studied so far. Terpenes, the volatile compounds responsible for much of the flower’s aroma, have shown more variable results.
- A 2016 study of gamma irradiation in medical cannabis examined four varieties and found unchanged THC and CBD levels alongside reductions in some individual terpenes. Affected compounds generally fell by 10% to 20%, with individual reductions reaching 38%. Those figures describe particular terpenes, rather than a loss of 38% of the total terpene content.
- A separate 2023 study examining gamma irradiation and cannabis composition found little effect on terpenes under its tested conditions.
One should be cautious about translating either result into a claim about how well a medicine works. These were laboratory measurements, and they do not establish any differences in symptom relief between irradiated and non-irradiated medical cannabis.
There is also the question of substances produced by mould before treatment. Some fungi produce mycotoxins, which are chemical compounds rather than living organisms. In a 2025 study, researchers detected residual mycotoxins and recovered viable fungi after gamma irradiation, despite substantial reductions in microbial counts. The findings apply to the samples and treatment studied, but illustrate why testing for viable fungi and testing for their toxins answer different questions.
Why do microbial standards and prevention matter?
Historically, the vast majority of medical cannabis flower prescribed in the UK has been imported, with Canada among its principal suppliers. UK cultivation, including our work at Glass Pharms, is beginning to broaden that supply. Overseas production remains central to the market, however, so differences in microbial standards between countries directly affect how flower is prepared for UK patients.
The British Pharmacopoeia (BP) sets quality standards for UK medicines and incorporates European Pharmacopoeia standards. In the illustration below, dish one represents the inhalation requirements in BP 5.1.4. Dishes two and three represent the herbal-medicine categories B and C in BP 5.1.8. Their maximum acceptable yeast-and-mould counts are 20, 500 and 50,000 colony-forming units per gram (CFU/g), respectively, including each chapter’s counting conventions.
Dish four represents the Canadian comparison. Health Canada accepts herbal-product microbial standards for dried cannabis intended for smoking or vaping, including specifications permitting up to 50,000 CFU/g. This can overlap with category C rather than represent a separate, higher limit. For UK supply, the MHRA expects alignment with inhalation requirements, with deviations requiring justification. Flower accepted in its original market may therefore need further treatment before it can be legally supplied to UK patients.
Illustrative comparison of microbial specifications, not actual test results. The number, size, and appearance of colonies do not represent exact counts or typical contamination levels.
Irradiation can provide that microbial reduction. Its implementation does not tell us the final fungal count, however, just as a non-irradiated label does not establish that a batch meets any particular standard. Both require the same level of regulatory oversight and testing.
The flower must then remain stable during transport and storage. Any surviving microorganisms still have nutrients available and may multiply if sufficient water becomes available. Producers need to control water activity, a measure of the water available for microbial growth, through drying and maintain it at a sufficiently low level during packaging and storage. Irradiation does not remove that requirement.
At Glass Pharms, we do not irradiate our medical cannabis flower. We work to prevent contamination through strict hygiene and regulatory practices during cultivation and handling, use controlled drying and curing to limit water available for microbial growth, and test throughout cultivation and post-harvest processing to verify these measures are effective.
