What tirzepatide is
Tirzepatide activates two incretin receptors — GIP and GLP-1. Incretins are gut hormones released after eating that signal insulin release, slow gastric emptying and reduce appetite. Targeting both receptors produces larger effects on blood glucose and body weight than targeting GLP-1 alone.
It is sold as Mounjaro for type 2 diabetes and weight management, and as Zepbound in some territories. It is a licensed medicine in the UK.
Why this page is different
Most pages on this site exist because the evidence is weaker than the marketing. This one is the opposite. Tirzepatide has been through the full regulatory process: large randomised controlled trials, independent regulatory review, published safety data, ongoing pharmacovigilance.
The question with tirzepatide is not whether the molecule works. It is whether the vial in front of you contains it.
That reframing matters, because it changes what you should actually be worried about.
Prescription-only means something
Tirzepatide is a prescription-only medicine in the UK. It can be supplied lawfully only against a prescription from a qualified prescriber, dispensed through a registered pharmacy.
Advertising a POM to the public is prohibited under the Human Medicines Regulations 2012. Supplying one without a prescription is a criminal offence, not a technicality.
The prescription requirement is not bureaucratic obstruction. It exists because the drug interacts with other medicines, is contraindicated in specific conditions, requires dose titration to manage gastrointestinal effects, and needs monitoring in people with diabetes to avoid hypoglycaemia when combined with insulin or sulfonylureas.
None of that assessment happens when a vial arrives in the post.
The counterfeit problem
Tirzepatide's popularity has made it one of the most heavily counterfeited medicines in circulation. Falsified pens and vials have been recovered in the UK supply chain, and people have been hospitalised after using them.
Counterfeit GLP-1-class products fail in several distinct ways, and each has caused documented harm:
- Wrong drug entirely. Insulin has been found in products sold as GLP-1 agonists. Someone without diabetes injecting insulin can become severely hypoglycaemic very quickly.
- Wrong concentration. Overstated or understated strength, with no way to detect it before use.
- Non-sterile preparation. Injection-site infections and worse, unrelated to the active ingredient.
- Nothing active at all. The least dangerous failure, and probably the most common.
Sophisticated counterfeit packaging is now routine. Holograms, batch numbers and authentic-looking labels are all reproducible. Visual inspection is not a safety check.
Known adverse effects
Because tirzepatide is licensed, its side-effect profile is properly characterised rather than guessed at. Gastrointestinal effects — nausea, vomiting, diarrhoea, constipation — are common, particularly during dose escalation. Pancreatitis and gallbladder disease appear in the safety labelling. Rodent studies showed thyroid C-cell tumours, which is why the licensed product carries specific contraindications around medullary thyroid carcinoma and MEN 2.
These are manageable when someone is titrating under supervision with the correct product. They are considerably harder to manage alone, with material of unknown strength, and no prescriber to call.
Where this leaves you
If you want tirzepatide, the evidence supports wanting it. It also supports getting it through a route where somebody has verified what is in the vial, checked it against your medical history, and remains contactable if something goes wrong. That route exists and is not difficult to access.
The grey market offers the same molecule name at lower cost, with none of the verification. Given that the counterfeit rate in this specific drug class is high and the failure modes include acute hypoglycaemia, that is a poor trade.