Tirzepatide vs Semaglutide for Weight Loss: What the Trial Data Shows

The trial results that started the comparison

The question of tirzepatide versus semaglutide for weight loss moved from theory to evidence in 2025, when SURMOUNT-5 (a 72-week, open-label randomised trial of 751 adults with obesity and no diabetes) put the two medicines directly against each other. Tirzepatide produced a greater average reduction in body weight than semaglutide 2.4 mg. The New England Journal of Medicine published the results, and NICE's appraisal of tirzepatide (TA1026) acknowledged that indirect comparisons across the trial programme also favoured tirzepatide.

The earlier individual trials are worth understanding too. SURMOUNT-1 (2,539 adults, 72 weeks) recorded around 20–21% average weight loss at tirzepatide 15 mg. STEP 1 (semaglutide 2.4 mg, 68 weeks) recorded roughly 15% average weight loss, as published in the New England Journal of Medicine. Neither trial compared the two medicines directly, which is exactly why SURMOUNT-5 mattered.

One further development worth noting: the MHRA approved a higher 7.2 mg semaglutide dose (Wegovy) in early 2026, with trials reporting around 20.7% average weight loss over 72 weeks at that dose, narrowing the gap with tirzepatide 15 mg considerably. Specific dose and pen availability is always confirmed at the time of your consultation.

A note on medicines sometimes confused with these: Ozempic is also semaglutide, but it is licensed for type 2 diabetes in the UK, not weight loss. The same applies to the oral tablet Rybelsus. If you are researching Mounjaro versus Ozempic, the licensing difference is the first thing to understand.

How the two mechanisms translate into practice

Tirzepatide activates two gut-hormone receptors: GIP and GLP-1. Semaglutide activates one: GLP-1. Both slow gastric emptying and reduce appetite; the dual action of tirzepatide is the leading scientific explanation for why its trial results generally sit higher. That said, mechanism alone does not predict how any individual will respond, which is why a prescriber assesses the whole picture rather than simply pointing at a headline figure.

Side effects share a similar profile for both. Nausea, constipation, diarrhoea, indigestion and fatigue are the most commonly reported, typically peaking after starting treatment or after a dose increase and settling within days to a couple of weeks for most people. The NHS patient information pages for tirzepatide and semaglutide set these out clearly and are worth reading before treatment.

One practical difference relates to oral contraception. For tirzepatide specifically, UK guidance advises adding a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because pill absorption may be reduced. The same evidence base does not apply to semaglutide. If HRT is relevant to you, NHS England also suggests considering transdermal preparations during tirzepatide treatment. These are the kinds of clinical details a prescriber covers at consultation, not things to resolve from a comparison article.

For a more detailed look at how the difference in results translates into kilograms, the tirzepatide vs semaglutide weight-loss difference in kg page works through the numbers from the published trials.

Eligibility, access and where cost fits in

Both medicines are licensed for 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 guidance. A BMI figure on its own never guarantees a prescription, the prescriber considers your full health picture, current medicines and any contraindications.

On the NHS, tirzepatide is being rolled out in phases under NICE TA1026, starting with the highest-need cohorts. Semaglutide (Wegovy) is available via specialist weight management services under TA875, though waiting lists are commonly long. Neither route is quick for most people. Private treatment through a regulated pharmacy is the alternative, subject to the same clinical assessment. If you want a fuller picture of how the two options compare on availability and clinical profile, the semaglutide vs Mounjaro for weight loss page brings those threads together. You can also read more about Wegovy versus Mounjaro from a cost and access perspective if that is your next question.

Cost varies by dose and provider. Prices for both medicines have shifted over 2025, Eli Lilly raised Mounjaro's UK list price from September 2025, with widely reported private market prices ranging roughly £149–£375 per month depending on dose and pharmacy. If budget is a deciding factor, the Wegovy vs Mounjaro cost comparison page sets out what those numbers mean in practice. At nume, one transparent price covers the consultation, prescription, treatment and next-day delivery, there are no separate fees stacked on top. We are not the cheapest option available; that is a deliberate position, because for a prescription medicine, the right question is not which listing is lowest.

If you are already on one medicine and wondering whether switching makes sense, or if a payday order in the first week of the month means you are mid-cycle on a dose increase, those are exactly the questions to raise with a prescriber at your next review, not something to act on without clinical input. Our FAQs cover some of the most common questions about switching and titration.

The clinical team at nume reviews every consultation personally. Which of these medicines suits you is a decision our prescribers make with you, based on your health history, current medicines and goals, not based on trial headlines alone. Check your eligibility by starting your free consultation.