How the MHRA classifies a peptide: medicine, "special", or unauthorised product

UK law doesn't have a grey area for peptides — a product is a medicine, a lawful unlicensed "special", or something being sold outside the rules entirely, decided by what it's actually for, never by what its label says. This page explains the legal test the MHRA applies, not the enforcement record that follows from it.

The test that decides it: function, not label

The Human Medicines Regulations 2012 define a "medicinal product" two ways, and a substance only needs to meet one of them. Under regulation 2, it's a medicine by presentation if it's described as having properties for treating or preventing disease — or by function if it may be administered to a person with a view to restoring, correcting or modifying a physiological function by a pharmacological, immunological or metabolic action (or to making a medical diagnosis). This is the UK's own domestic restatement of a definition it inherited from EU law, and the same two-part test still underpins how the MHRA treats a synthetic peptide sold to a UK buyer today.

The practical effect: a supplier can't contract out of medicines law by choosing different words. If a peptide is sold with a suggested human dose, testimonials about its effect on the body, or reconstitution supplies for injection, the MHRA can treat that marketing as evidence the product meets the function or presentation test — a "research use only" or "not for human consumption" disclaimer on the same page doesn't override it. What does "Research Use Only" actually mean? covers the equivalent US mechanism (21 CFR 201.128's "intended use" doctrine) in detail; the UK's function/presentation test reaches the same practical conclusion by a different statutory route.

The two lawful routes to a UK peptide medicine

Once something is a medicinal product, UK law gives it exactly two lawful ways to reach a patient. The ordinary route is a marketing authorisation (MA): a full MHRA review of safety, quality and efficacy before the product can be sold at all, the same approval pathway that put semaglutide and tirzepatide on UK shelves as prescription-only medicines (POMs) — Mounjaro's type 2 diabetes authorisation came in September 2022, extended to weight management in November 2023. Nothing sold as a "research peptide" to a UK consumer has been through this review.

The second, much narrower route is what's commonly called a "special" — the subject of the next section, and the one grey-market sellers most often gesture at without actually qualifying for it.

Why "specials" isn't a backdoor for grey-market peptides

Regulation 167 of the Human Medicines Regulations 2012 exempts an unlicensed "special medicinal product" from needing a marketing authorisation, but only under conditions built for one-off clinical need, not routine retail supply. To qualify, the product must be manufactured or assembled to a specific prescriber's specification, supplied in response to a genuine unsolicited order, and intended for a named patient whose treatment that prescriber is directly responsible for — to meet that patient's special clinical need. MHRA guidance is explicit that cost, convenience or simple unavailability of a preference don't count as a special need, and the exemption isn't available at all where a licensed, pharmaceutically equivalent product already exists on the UK market.

The manufacturer or importer side is regulated too: making a "special" in the UK requires the manufacturer to hold its own Manufacturer's "Specials" (MS) Licence, with the site inspected by the MHRA for Good Manufacturing Practice under a named quality controller and production manager. A vial shipped to a UK buyer from an unlicensed overseas seller, with no prescriber, no named patient and no MS-licensed manufacturing behind it, doesn't meet a single element of this exemption — it simply isn't what "specials" describes, whatever a seller's FAQ page claims.

Who decides a genuinely borderline case

For products that sit closer to a genuine edge case — is this a cosmetic, a food supplement, a medical device, or a medicine? — the MHRA runs a formal borderline-classification process, set out in Part 9 of the Human Medicines Regulations 2012 and explained in MHRA Guidance Note 8, "A guide to what is a medicinal product." A manufacturer or importer can request a formal determination directly from the MHRA's borderline products team, and the agency updated this guidance again in October 2025, adding further worked examples across categories including CBD products and faecal microbiota transplantation. An injectable peptide marketed for a human physiological effect is not, in practice, a genuinely close borderline case under this framework — it falls squarely inside the function test in section 1 above — but the existence of a formal determination route is itself evidence that "we're not sure what category this is" isn't a legal status a seller gets to claim for themselves; it's a request they can make to the regulator, and one the grey market for injectable peptides has not been making.

What getting this wrong actually risks

Selling an unlicensed medicinal product outside both lawful routes above is a criminal offence under the Human Medicines Regulations 2012, not merely a licensing technicality — on conviction on indictment, it carries a fine, imprisonment for up to two years, or both. This isn't theoretical for this exact category: this site's own index of UK enforcement actions already documents an MHRA Criminal Enforcement Unit prosecution ending in prison sentences, and a series of raids since October 2025 that seized unlicensed retatrutide and tirzepatide pens under these same regulations. That page covers what's actually happened; this one explains the legal test those actions were applying.

If you're offered a peptide in the UK

Ask which of the two lawful routes above the product is actually on: does it carry a UK marketing authorisation (check the packaging for a PL number, or ask your pharmacist), or has it genuinely come through a named prescriber and an MS-licensed "specials" manufacturer for your own individual clinical need? If neither applies, the seller's own description of the product — "research use only," "not for human consumption," or otherwise — has no bearing on whether it's a legal medicine in the UK. See are peptides legal? for the equivalent breakdown across other countries, and UK enforcement actions for the specific cases this framework has already produced.