Mounjaro and Semaglutide: Are They the Same Thing?

Two different drugs, two different mechanisms

Semaglutide is a GLP-1 receptor agonist. It mimics a gut hormone called glucagon-like peptide-1, which slows gastric emptying, reduces appetite and helps regulate blood sugar. The UK's licensed weight-management brand is Wegovy, made by Novo Nordisk. Ozempic is also semaglutide, but if you are wondering whether semaglutide is the same as Mounjaro, it is licensed solely for type 2 diabetes (not weight loss) and the two should never be conflated.

Tirzepatide, sold as Mounjaro by Eli Lilly, does something different. It activates two gut-hormone receptors simultaneously: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). Because of this dual action, Mounjaro is the only weight-loss medicine of its kind currently licensed in the UK. That additional GIP pathway appears to amplify the appetite-reducing and metabolic effects beyond what a GLP-1 alone produces. You can read a deeper look at how each works in our guide on whether tirzepatide and semaglutide are the same thing.

So to answer the headline question directly: Mounjaro and semaglutide are not the same molecule, not the same mechanism and not interchangeable at the pharmacy counter.

What the trial data show, side by side

Both medicines have strong clinical trial evidence behind them, published in the New England Journal of Medicine. The SURMOUNT-1 trial found that tirzepatide produced an average body-weight reduction of around 20–21% over 72 weeks at the highest dose tested. The STEP 1 trial found that semaglutide 2.4mg produced an average reduction of around 15% over 68 weeks. Neither figure translates directly to what any individual will experience, and both trials involved lifestyle support alongside medication.

The most direct comparison came from the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, which set tirzepatide against semaglutide 2.4mg head-to-head in 751 adults with obesity and no diabetes over 72 weeks. Tirzepatide produced greater average weight reduction. NICE discussed this evidence in the committee papers for tirzepatide's appraisal, TA1026. The gap narrows somewhat with the newer Wegovy 7.2mg dose, approved by the MHRA in 2026, which reported around 20.7% average weight loss in its own trials, closer to Mounjaro's top-dose figures. A trial figure is a population average, not a personal forecast; individual response varies widely with both medicines.

For a closer look at what these medicines cost in the private market, our cost comparison page covers what private pricing typically includes and what to watch for.

Side-effect profiles, similar in pattern, not identical

Both medicines share a gastrointestinal side-effect profile because both slow gastric emptying. Nausea, loose stools, constipation, reflux and fatigue are the most commonly reported effects with either treatment, and they tend to be most noticeable after starting or after a dose step up. For the majority of people they settle within days to a couple of weeks. A gradual titration schedule is the main tool prescribers use to manage this.

The NHS's patient information pages for tirzepatide and for semaglutide set out the full side-effect lists for each medicine respectively, including less common effects and situations requiring urgent medical attention. In January 2026 the MHRA issued a Drug Safety Update flagging acute pancreatitis as an infrequent but serious risk with GLP-1 medicines; severe stomach pain spreading to the back warrants prompt medical advice with either drug.

One practical difference worth noting: for women using oral contraceptive pills, NHS guidance advises adding a non-oral method for the first four weeks of Mounjaro and for four weeks after each dose increase, because tirzepatide's effect on gastric emptying may reduce pill absorption. There is no equivalent evidence of that interaction with semaglutide. Our Wegovy versus Mounjaro comparison goes into further detail on these clinical differences, and our clinical team can answer specific questions through aftercare support.

Which one should you consider, and how to find out

This is precisely the question a prescriber is trained to help you answer. The right treatment depends on your BMI, your weight-related health conditions, your current medicines, your personal history and how you respond to a starter dose. Some people do better on one mechanism than the other, and some find one pen's injection device easier to use than the other. What arrives, practically speaking, is a discreet DPD parcel tracked to your door, but what's inside needs to be the right prescription for you specifically, not just the one with the largest trial headline.

NICE recommends tirzepatide (TA1026) for adults with a BMI of 35 or above alongside at least one weight-related condition; it also recommends semaglutide (TA875) under similar criteria, though within a specialist weight management service and for a maximum of two years under NHS commissioning rules. Private eligibility follows the licensed criteria, which are broader. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, a detail covered in full on our treatment overview page.

If you are still working out which medicine might suit you, it helps to read about how Mounjaro and Wegovy relate more specifically, including the branded question many people search: our page on whether Mounjaro and Wegovy are the same thing addresses that directly. When you are ready to move forward, the next step is a free consultation with our prescribers. Check your eligibility and one of our team will review your answers the same day.