How is UK medical cannabis tested, irradiated and released?

This is the part of the system that most clearly separates a prescribed medicine from anything bought illicitly, and it is also the part patients hear least about. It is worth knowing what has happened to a product before it reaches you.

Why any of this is necessary

Because plant material is a biological product, and biological products carry risks that a purified molecule does not: mould and bacteria from growing and drying, residues from cultivation, and variation between harvests.

The people receiving these medicines include patients whose immune systems are suppressed by other treatment, for whom inhaling fungal spores is a serious risk rather than a theoretical one. That single fact drives most of the processing described below.

It is also the substantive difference between prescribed and illicit supply, which is set out in How is medical cannabis different from street cannabis?

What a certificate of analysis covers

A certificate of analysis is the laboratory record for one specific batch. It states what was tested, by what method, and what was found.

• Cannabinoid content, so the preparation matches its specification
• Microbial testing, including yeasts, moulds and specified bacteria
• Pesticide residues
• Heavy metals, which plants can take up from growing medium
• Residual solvents, where an extraction process has been used
• Moisture and, for some products, foreign matter

Your pharmacy holds this information, and a batch number on the container links what you have to the specific record. There are glossary entries for certificate of analysis and batch number.

Why some flower is irradiated

Irradiation is used to reduce microbial contamination to a level acceptable for a medicine. It does not make the product radioactive, and it is used across food and pharmaceutical manufacturing for the same reason.

Dried plant material cannot be heat-sterilised without destroying it, so where microbial levels exceed the standard, irradiation is one of the few options. Both electron beam and gamma methods are used.

Patients do raise a fair objection: there is evidence that irradiation can reduce terpene content, and some people report a difference in smell and taste. Whether that translates into a difference in effect is not well established, and the trade-off is being made against a documented contamination risk rather than a theoretical one.

Not every product is irradiated. If it matters to you, it is a reasonable thing to ask your clinic or pharmacy about a specific preparation, and there is a glossary entry for irradiation.

Batch release, import and why supply changes

Before a batch can be supplied in the UK it has to be released, which is a formal check that it was made to the required standard and meets its specification. Unlicensed products are also imported under licence.

That combination — per-batch release plus per-consignment import licensing, on top of a Home Office controlled drug licence — is why supply of a particular preparation is less predictable than for an ordinary licensed medicine. A delay anywhere in that chain shows up as a product being unavailable.

It is the most common practical frustration patients report, and it is a structural feature of the specials route rather than a failing of any one clinic. The background is in Why is medical cannabis an unlicensed medicine in the UK?

Flower, granulate and extracts

The same quality requirements apply whatever the form, but the processing differs. Granulated preparations are milled plant material, which changes handling and dosing consistency rather than the underlying medicine.

There are glossary entries for flos, granulate and full spectrum. How each is used is covered in How are cannabis-based medicines taken?

What testing does and does not tell you

Testing and release confirm that a batch is what it says it is and is within accepted limits for contamination. That is a great deal, but it is worth being clear about what it does not do.

What it establishes is identity and quality: that the cannabinoid content matches the specification, and that microbial contamination, pesticides, heavy metals and, where relevant, residual solvents are within acceptable limits. It also makes the product consistent from one batch to the next, which is part of why a prescribed preparation behaves more predictably than anything from an unregulated source.

What it does not do is tell you whether a medicine will help your condition, or how you personally will respond. That depends on the condition, the dose, the route and individual factors, and it is judged over time by your specialist. Testing removes a category of risk; it does not turn an unlicensed medicine into a guaranteed treatment, and the evidence base remains limited for many uses.

What this means for you as a patient

Most of this happens before your medicine reaches you, so there is little you need to do — but a few things are worth knowing.

• The batch number is your link to the record. If you ever have a question about a specific container, that number lets your pharmacy trace it back to its certificate of analysis.
• You can ask about a specific preparation. Whether something has been irradiated, for instance, is a reasonable question to put to your clinic or pharmacy.
• Report a problem. If you suspect a quality issue or an unexpected reaction, tell your clinic and pharmacy; suspected side effects can also go to the MHRA, as set out in side effects of cannabis-based medicines.
• Expect occasional supply gaps. They are a structural feature of the specials route rather than a sign of a problem with your clinic, and planning repeats early softens them.
• Keep the labelled packaging. It carries the information that ties your medicine to its batch and to you, as covered in how medical cannabis is dispensed and delivered.

Questions worth asking your clinic or pharmacy

If quality and consistency matter to you, these are reasonable things to ask:

• Has this preparation been irradiated, and does that affect what I should expect?
• Can I see or ask about the certificate of analysis for my batch?
• How consistent is this product likely to be from one supply to the next?
• How do you handle a shortage if my usual preparation is unavailable?
• Who do I contact if I think there is a problem with a batch?

A clinic or pharmacy that answers these openly is a good sign; this is information you are entitled to understand, and asking about it is a normal part of being a well-informed patient rather than a difficult one.

Patient guide · Written by the Flora House editorial team · Last reviewed: 7 October 2026