Mounjaro or tirzepatide: which is better, and is there actually a difference?

Step 1: settle the name confusion before anything else

This is a question our prescribers hear most weeks, and it's a sensible one given how the medicine is marketed. Tirzepatide is the International Non-proprietary Name (INN), the generic, scientific name for the active compound. Mounjaro is the brand under which Eli Lilly sells it in the UK and most of the world. When you buy the branded pen in a UK pharmacy, the active ingredient listed on the label is tirzepatide. There is no generic tirzepatide pen available in the UK; Mounjaro is currently the only licensed form.

This matters practically. If you see a product described as "tirzepatide" without the Mounjaro branding, sold online at a price that seems low, treat that as a serious red flag. The MHRA has warned repeatedly about counterfeit and unlicensed weight-loss pens sold through social media and unofficial websites. You can verify any online pharmacy on the GPhC register before handing over any personal or payment information. Legitimate supply in the UK comes through the licensed supply chain, dispensed by a GPhC-registered pharmacy on a valid prescription.

So: Mounjaro or tirzepatide, identical. The real question is whether tirzepatide suits you, and how it stacks up against other licensed options, including newer contenders, which is where our guide on what's better, Mounjaro or retatrutide, is worth bookmarking for later. That's where it gets interesting.

Step 2: understand what tirzepatide actually does (and why the dual mechanism matters)

Most GLP-1 medicines work by activating a single receptor (the glucagon-like peptide-1 receptor) which reduces appetite, slows the movement of food through the stomach, and helps regulate blood sugar. Tirzepatide does that too, but it also activates a second receptor: GIP, or glucose-dependent insulinotropic polypeptide. Both pathways are involved in appetite signalling and metabolic regulation.

Whether the dual action explains the clinical results is still being studied, but the results themselves are well-documented. In the SURMOUNT-1 trial published in the New England Journal of Medicine, adults with obesity taking tirzepatide 15 mg lost an average of around 20–21% of body weight over 72 weeks, alongside a reduced-calorie diet and increased activity. That is a meaningful figure. You can read the NHS summary of tirzepatide for a clear, patient-level overview of how the medicine works and what side effects to expect.

The practical upshot: tirzepatide is not simply a stronger version of existing GLP-1 medicines, it has a different mechanism. Whether that makes it the right choice for a given person is a clinical judgement, not a marketing one. Thinking about how to get the most from treatment is worthwhile too; how to make Mounjaro work better covers the lifestyle factors that sit alongside the prescription.

Step 3: compare tirzepatide honestly against semaglutide, the main alternative

The most meaningful comparison is between tirzepatide (Mounjaro) and semaglutide 2.4 mg (Wegovy), the other injection licensed for weight management in the UK. Both are once-weekly subcutaneous injections. Both carry a titration schedule starting at a lower dose to manage tolerability before working up to the maintenance dose. Both are prescription-only.

On average weight loss, tirzepatide has shown greater reductions in head-to-head evidence. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, compared the two medicines directly in adults with obesity and no diabetes: tirzepatide produced greater average weight loss over 72 weeks than semaglutide 2.4 mg. The gap narrows at semaglutide's newer 7.2 mg dose, approved by the MHRA in 2026, but tirzepatide's results remain strong across the licensed dose range.

That does not mean tirzepatide is automatically the better choice for every individual. Side-effect profiles are broadly similar (both are GI-led) but some people find one more manageable than the other. Medical history, other medicines, and individual response all feed into the clinical decision. If you're weighing up the cost side of this comparison, the Mounjaro cost page gives context on UK private pricing. For a broader look at how the two medicines sit in the licensed landscape, the weight-loss treatment overview covers both options.

Step 4: eligibility, and where a prescriber fits in

The UK licence for tirzepatide covers adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. BMI alone does not determine approval, the prescriber looks at the whole picture, including current medicines, medical history, and any contraindications.

NICE's recommendation (TA1026) sets narrower criteria for NHS access: broadly, a BMI of 35 or above plus at least one qualifying comorbidity, via a specialist weight management service. Private prescribing follows the licensed criteria, which are wider. Neither route bypasses clinical assessment; a prescriber must be satisfied that the treatment is appropriate before any pen is dispensed.

At nume, every consultation is read by a GPhC-registered Independent Prescriber on the same day, a real clinician, not automated software. If you'd like to understand more about the full tirzepatide picture before starting, the tirzepatide information page and the Mounjaro overview are both worth reading alongside this one. Emerging medicines like retatrutide are also attracting attention; our page on retatrutide versus tirzepatide sets out what the current evidence does and doesn't show. When you're ready to take the next step, check your eligibility with a free consultation.