Which medicines have tirzepatide in them, and what does it actually do?

You've seen the name tirzepatide, but you're not sure what's actually in the pen

That's a fair place to start. The name appears in news articles, NICE guidance, and pharmacy listings, yet the box in most people's fridge says something different. The short answer is that tirzepatide is a synthetic peptide molecule (a string of amino acids engineered in a laboratory) and Eli Lilly markets it under the brand name Mounjaro. The pen you inject once a week, the one that sits in your fridge door, contains tirzepatide as its sole active ingredient alongside a small volume of solution to carry it.

A question our prescribers hear most weeks is whether cheaper or alternative products sold online contain the same thing. They do not. In the UK, tirzepatide is protected by patent, and Mounjaro is the only licensed medicine that contains it. If you come across a product described as "tirzepatide" that is not Mounjaro dispensed through a regulated pharmacy, that is a serious safety concern, the MHRA has warned publicly about counterfeit weight-loss pens circulating online, some containing entirely different substances. You can read more about what tirzepatide is and why the source matters on our dedicated page.

The NHS medicines page for tirzepatide is a straightforward starting point if you want a patient-level overview from a Tier-1 UK source: nhs.uk/medicines/tirzepatide.

Why tirzepatide works the way it does, the two receptors explained plainly

Most GLP-1 medicines activate a single gut-hormone receptor. Tirzepatide activates two: the GIP receptor and the GLP-1 receptor. Both are involved in regulating appetite and blood-sugar levels after you eat. Activating them together appears to produce a larger reduction in appetite and body weight than either pathway alone, which is why head-to-head trial evidence (including the SURMOUNT-5 study published in the New England Journal of Medicine in 2025) found tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks.

The mechanism also slows gastric emptying, which means food moves through the stomach more slowly and feelings of fullness last longer after a meal. That change in appetite is what most people notice first. It is not a stimulant and does not work by speeding the metabolism.

Because the molecule is a peptide, it cannot survive digestion in the gut intact, which is why it is injected rather than swallowed. Each Mounjaro KwikPen delivers one week's dose under the skin of the abdomen, thigh, or upper arm. If you are curious about what tirzepatide is made of at a molecular level, that page goes into the chemistry in more detail.

Mounjaro's six strengths: the same ingredient, different amounts

All six Mounjaro pen strengths (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg) contain tirzepatide. The starting 2.5 mg strength exists to let the body adjust gradually; it is a tolerability step rather than the dose at which most clinical benefit is seen. A prescriber typically steps the dose up in roughly four-week intervals, guided by how well the treatment is tolerated.

The licence covers adults with a BMI of 30 or above, or 27 or above when a weight-related health condition such as high blood pressure, high cholesterol, or type 2 diabetes is present. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A prescriber assesses all of this at consultation, BMI alone does not decide suitability. You can explore the full uses of tirzepatide including its diabetes licence on a dedicated page.

It is also worth knowing that tirzepatide and Mounjaro are the same thing in the sense that every Mounjaro pen contains tirzepatide, but the distinction matters legally and practically, particularly if you are comparing information from different countries. The US brand Zepbound also contains tirzepatide but does not exist as a UK product.

For context on how private treatment is priced, including what happened to Mounjaro's UK list price in 2025, our Eli Lilly Mounjaro price page sets out what changed and why. Cost context is one piece of a bigger picture; the more important question is whether the supply chain is legitimate. NICE's appraisal of tirzepatide, TA1026, is the UK clinical guidance that underpins prescribing decisions.

Getting access to tirzepatide the right way

Because tirzepatide is prescription-only, the legal route in the UK is straightforward: a prescriber assesses whether it is suitable for you, issues a prescription if it is, and a GPhC-registered pharmacy dispenses it. There is no legitimate shortcut.

Private online pharmacies, including nume, follow this path. A GPhC-registered Independent Prescriber reviews your consultation (a real clinician, not software) on the same day. If clinically appropriate, a prescription is issued and the pen is dispensed and dispatched. NHS access is also possible for those who meet the eligibility criteria under NICE TA1026, though the rollout is phased and criteria are strict.

If you would like to understand how different weight-loss treatments compare, including how tirzepatide sits alongside other options, that page covers the landscape without assuming you have already made a decision. When you are ready to find out whether tirzepatide is suitable for your situation, you are welcome to start your free consultation with our prescribers.