Tirzepatide or semaglutide: which is best for weight loss?

What the trial data actually shows

Both medicines reduce appetite and slow gastric emptying, but they work through different pathways. Semaglutide (Wegovy) activates the GLP-1 receptor only. Tirzepatide (Mounjaro) activates both GLP-1 and GIP receptors, making it the only dual-agonist weight-loss medicine licensed in the UK. That pharmacological difference appears to translate into real-world results.

In SURMOUNT-1, tirzepatide at its highest dose produced an average body-weight reduction of around 20–21% over 72 weeks in adults with obesity — figures cited directly in NICE's appraisal of tirzepatide (TA1026). The STEP 1 trial for semaglutide 2.4mg reported around 15% average reduction over 68 weeks, as documented in the NEJM STEP 1 paper. The SURMOUNT-5 head-to-head then compared the two medicines directly, and tirzepatide again produced the greater average loss.

One caveat worth knowing: Wegovy now has a licensed 7.2mg dose, approved by the MHRA in April 2026, which in trials produced around 20.7% average weight loss over 72 weeks. That narrows the gap considerably. The higher semaglutide dose is newer, and long-term real-world data are still accumulating. It also carries a BMI threshold of 30 or above only, with no lower-BMI indication at that dose. If you want a detailed breakdown of how the two medicines compare on efficacy, our page on semaglutide or tirzepatide walks through the evidence side by side. The comparison is genuinely moving, which is part of why the decision belongs with a prescriber rather than a comparison table.

Eligibility: who each medicine is licensed for

The licensed eligibility criteria are similar but not identical. Tirzepatide is licensed for adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Semaglutide (Wegovy) carries the same broad thresholds. For both medicines, BMI thresholds are 2.5 kg/m² lower for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds under UK clinical guidance.

NICE recommends tirzepatide under TA1026 for NHS use primarily in adults with a BMI of 35 or above and at least one weight-related comorbidity. NICE recommends semaglutide under TA875 for NHS use only within specialist weight management services and for a maximum of two years. NHS criteria are strict and access is phased. Private treatment through a regulated pharmacy does not require you to meet NHS-specific thresholds, but a prescriber still assesses the full clinical picture before approving anything. The consultation process at nume covers both medicines and works through your eligibility with you directly.

One point that sometimes catches people out: Ozempic is also semaglutide, but it is licensed for type 2 diabetes, not weight loss. If you have seen Ozempic mentioned in weight-loss conversations, it is worth reading our explainer on Ozempic, tirzepatide and semaglutide before drawing conclusions.

Side effects and tolerability

The side-effect profiles of tirzepatide and semaglutide are broadly similar because both slow digestion. Nausea, loose stools, constipation, reflux and fatigue are the most commonly reported effects for both, and they tend to be most noticeable after starting or moving up a dose, often settling within a couple of weeks. The graduated dosing schedules used for both medicines exist largely to reduce how pronounced these effects feel early on.

There are nuances. Tirzepatide's dual mechanism means GIP activation may reduce the nausea some people experience with GLP-1 agonists alone, though responses vary individually. On the interaction side, women taking oral contraceptives need to use an additional non-oral method for the first four weeks of tirzepatide treatment and for four weeks after each dose increase, because absorption of the pill may be reduced. NHS guidance does not flag the same concern for semaglutide. The full safety picture for both medicines, including what to watch for urgently, is covered in our safety comparison page.

For pancreatitis specifically: the MHRA issued a Drug Safety Update in January 2026 highlighting acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. Severe persistent stomach pain reaching the back is a reason to seek urgent medical attention on either treatment.

Cost and practical differences

On cost, Eli Lilly raised Mounjaro's UK list price from September 2025, with the highest-dose pen moving from around £122 to around £330) figures that have been widely reported. Private prices from pharmacies vary by dose and provider, typically in the £149–£375 range. Neither medicine is cheap; that reality applies to both. The price difference between Mounjaro and Wegovy depends on which dose you are on, which makes a direct comparison harder than it looks at first glance. For more on the brand-name medicines side by side, and a closer look at which is better, semaglutide or tirzepatide, our Wegovy vs Mounjaro page goes into detail.

Which medicine suits you is a clinical decision our prescribers make with you, based on your BMI, your health history and your preferences. Check your eligibility and a prescriber will review your answers the same day.