Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the two mechanisms differ, and why that matters for your decision
The single receptor Wegovy targets, and what it does
Wegovy contains semaglutide, a GLP-1 receptor agonist. GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after eating. It signals to the brain that food has arrived, stimulates insulin release, suppresses glucagon, and slows gastric emptying so meals take longer to clear the stomach. Semaglutide mimics this signal, extending and amplifying it well beyond what a natural meal would produce.
The practical result: appetite falls, portions feel satisfying sooner, and energy intake drops without requiring conscious restriction on the same scale. In the STEP 1 trial, published in the New England Journal of Medicine, adults using semaglutide 2.4mg alongside lifestyle changes achieved an average body-weight reduction of around 15% over 68 weeks. A higher 7.2mg dose has since received MHRA approval, with trial data reporting around 20.7% average loss over 72 weeks.
Because GLP-1 receptors are also present in the cardiovascular system, semaglutide has demonstrated benefits beyond weight, it holds a UK authorisation for reducing major cardiovascular event risk in eligible adults. For weight management, it is licensed for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition. The full clinical picture of Wegovy involves more than appetite alone.
The dual receptor Mounjaro activates, and why that distinction matters
Mounjaro contains tirzepatide, the only medicine licensed for weight management in the UK that activates two separate gut-hormone receptors: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). GIP is released from the upper small intestine shortly after food is absorbed and influences both insulin secretion and fat metabolism. The dual-agonist action is a meaningful mechanistic step beyond single-pathway medicines.
In practical terms, activating both receptors appears to produce a stronger appetite-suppressing effect than GLP-1 alone, though exactly how the two pathways interact continues to be studied. The SURMOUNT-1 trial found average body-weight reductions of around 20–21% at the 15mg dose over 72 weeks, and SURMOUNT-5 (a direct head-to-head against semaglutide 2.4mg, published in the NEJM in 2025) showed tirzepatide produced greater average weight loss. NICE's appraisal of tirzepatide (TA1026) references these results and notes that indirect comparisons favour tirzepatide.
Mounjaro also slows gastric emptying and, through GIP receptor activity, may influence fat cell behaviour more directly than semaglutide. It is the only dual GIP/GLP-1 receptor agonist available in the UK for weight management, which is worth understanding rather than just noting as a label.
How the mechanisms compare side by side
| Feature | Wegovy (semaglutide) | Mounjaro (tirzepatide) |
|---|---|---|
| Receptor pathway | GLP-1 only | GLP-1 + GIP (dual) |
| Administration | Once-weekly injection | Once-weekly injection |
| Average weight loss (licensed maintenance dose, clinical trials) | ~15% at 2.4mg (STEP 1, 68 wks); ~20.7% at 7.2mg (72 wks) | ~20–21% at 15mg (SURMOUNT-1, 72 wks) |
| Head-to-head evidence | Greater loss seen with tirzepatide (SURMOUNT-5, NEJM 2025) | Greater average loss vs semaglutide 2.4mg |
| UK weight-management licence | BMI ≥30, or ≥27 + weight-related condition | BMI ≥30, or ≥27 + weight-related condition |
| Additional UK authorisations | Cardiovascular risk reduction; MASH (liver disease, Jul 2026) | Type 2 diabetes management |
A question our prescribers hear most weeks is whether the dual mechanism makes Mounjaro automatically the right choice. The honest answer is that mechanism describes biology, not individual suitability. Side-effect profile, medical history, other medicines you take, and how your body responds in practice all shape which treatment makes clinical sense. You can explore the broader question of which suits different situations for more context.
What the mechanism difference means for the decision you're making
Both medicines are subcutaneous injections given once a week. Both produce GI-led side effects (nausea, constipation, and similar symptoms) most commonly in the early weeks of treatment or after a dose increase. The profile is similar enough that the mechanism alone does not predict how your stomach will respond. What it does suggest is that the ceiling for average weight loss is currently higher with tirzepatide, as the trial data shows.
Neither medicine is appropriate during pregnancy, whilst breastfeeding, or if you are trying to conceive. Neither is licensed for under-18s. Certain conditions (including a history of medullary thyroid carcinoma or pancreatitis) require careful prescriber review before either is considered appropriate.
It is also worth being clear about what is not in this comparison: Ozempic is semaglutide but holds a UK licence for type 2 diabetes management, not weight loss, and should not be conflated with Wegovy. If you are curious how Mounjaro and Ozempic compare, that distinction is central. Separately, if cost is part of your thinking, the reasons Wegovy is typically priced lower than Mounjaro are worth reading before you decide.
Which medicine suits you is a clinical decision our prescribers make with you, not one that mechanism alone can settle. If your thinking extends to how each medicine affects the body's inflammatory processes, our guide on whether Wegovy or Mounjaro is better for inflammation covers what the current evidence shows. Speak to our prescribers through a free consultation and they will review the full picture the same day.
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