Tirzepatide's different names: what they mean and why it matters in the UK

What exactly is an INN, and why does tirzepatide have one?

Every medicine has two layers of identity. There is the international non-proprietary name (INN), the agreed scientific name for the active molecule, which stays the same in every country and across every manufacturer. Then there are the brand names, which a pharmaceutical company chooses for its commercial product. Tirzepatide is the INN. Mounjaro is the brand Eli Lilly chose for the UK and most other markets; Zepbound is the brand they chose for the US. The molecule inside the pen is identical.

This distinction matters practically. When a prescriber, pharmacist or NHS guideline refers to tirzepatide, they mean the molecule, regardless of which product is being discussed. When you see Mounjaro on a prescription or a pharmacy label, that is tirzepatide. You can read about the UK-licensed product on our tirzepatide overview page if you want a fuller picture of how it works.

The NHS medicines page for tirzepatide uses the INN throughout, which is why some readers arrive there expecting to find a brand name and feel momentarily lost. The page covers exactly the same medicine as your Mounjaro pen.

So Mounjaro and Zepbound are the same drug, why do they have different names?

Pharmaceutical companies can apply for separate brand names in different regulatory territories. Eli Lilly filed Mounjaro as the brand for weight management and diabetes markets outside the US, and Zepbound for the US weight-management market specifically. The distinction is partly regulatory history and partly commercial strategy, not a clinical difference in the medicine itself.

For anyone in the UK, Zepbound simply does not exist as an obtainable product. It is not licensed by the MHRA, which is the body that authorises medicines in Britain. No UK pharmacy, including any regulated online pharmacy, can legally dispense it. Our page on how Zepbound differs from Mounjaro goes into greater detail on this distinction, but the short answer is: if you are in the UK and want tirzepatide, the product is Mounjaro, full stop.

It is worth knowing this if you come across listings that use the Zepbound name in a UK context. That is a red flag, not a quirky alternative source. The MHRA has warned publicly about counterfeit and unlicensed weight-loss medicines sold online, and unusual brand names or unfamiliar labelling are among the warning signs worth taking seriously.

Are there other names for tirzepatide, past, present or pipeline?

During its development, tirzepatide was referred to by a research code (LY3298176), which you might still encounter in older clinical papers. That is purely a development-phase identifier and has no relevance to what is dispensed today.

You may also have seen the name retatrutide mentioned alongside tirzepatide in research coverage. These are related but distinct molecules; our explainer on how retatrutide differs from tirzepatide covers this clearly. Retatrutide is a triple agonist still in clinical trials; it has no UK licence and is not available through any pharmacy at this stage.

There is also sometimes confusion between tirzepatide and semaglutide products. Semaglutide (sold as Wegovy for weight management and Ozempic for type 2 diabetes) is a different molecule working on a single receptor pathway. The two are not interchangeable, even though both fall under the GLP-1 medicines umbrella in public conversation. Our guide to different brands of tirzepatide separates these threads carefully if you want to compare.

Does the name on the box change anything about safety or effectiveness?

For the medicine itself, no. Tirzepatide, Mounjaro, Zepbound, they describe the same active molecule at the same concentrations. What differs is the regulatory context: a medicine licensed and dispensed through the UK supply chain has been through MHRA scrutiny and sits inside a chain of custody that can be traced. Something sold under an unfamiliar brand name, at a price that seems implausibly low, sourced outside that chain, carries a genuine risk of being fake, contaminated, or incorrectly dosed. The name on the box only means something when the box came from a legitimate source.

If you are already on treatment and want to understand why you might not yet be feeling the effects at your current dose, this page on not feeling different on Mounjaro is a practical read. And if you are thinking about your options more broadly, our treatment overview sets out what is licensed in the UK and how the different medicines compare. For an honest account of how our service works and who oversees it, our clinical team page is the place to start.

Tirzepatide in the UK costs more than it did before Eli Lilly revised its list price in September 2025; if cost is a factor in your thinking, our free consultation page is the right next step, it costs nothing to find out whether treatment is clinically appropriate for you, and a GPhC-registered prescriber, not a piece of software, reads every submission the same day it arrives.