The difference between Mounjaro and Wegovy, explained plainly

You've been told both options exist — here's what actually separates them

Picture the moment a prescriber asks which of these medicines you'd like to discuss. You know both involve a weekly injection. You know both are for weight management. Beyond that, the distinction gets muddy fast, and that's where most people land when they search this.

The core difference is biological. Wegovy works through a single receptor: GLP-1, a gut hormone that slows gastric emptying and reduces appetite. Mounjaro activates two receptors simultaneously (GLP-1 and GIP) making it the only dual-agonist weight-loss medicine licensed in the UK. In practice, both medicines reduce hunger and help you eat less; the dual mechanism in tirzepatide appears to produce a stronger average effect, though individual responses vary. A question our prescribers hear most weeks is simply: "which one works better?" The honest answer is that the head-to-head evidence favours Mounjaro on average, but if you want to understand what's the difference in Mounjaro and Wegovy from a clinical standpoint, "better" is always relative to your health history, tolerability and what your body does with each medicine.

If you want to read more about each medicine's mechanism before your consultation, the full Wegovy vs Mounjaro comparison on the nume site goes deeper on the pharmacology.

What the trial evidence shows, and what it can't tell you individually

In SURMOUNT-1, adults taking tirzepatide 15mg lost an average of around 20–21% of body weight over 72 weeks, published in the New England Journal of Medicine. The STEP 1 trial found semaglutide 2.4mg produced around 15% average weight loss over 68 weeks. Those figures are averages across thousands of participants, your result depends on factors no trial can pre-model for you.

The most direct comparison comes from SURMOUNT-5, a 72-week head-to-head trial published in 2025 in which tirzepatide produced greater average weight reduction than semaglutide 2.4mg in adults with obesity and no diabetes, and our detailed breakdown of the difference between Mounjaro and Wegovy covers what that trial means in practice. NICE referenced this evidence in its appraisal of tirzepatide, TA1026. Worth noting: Wegovy's newer 7.2mg dose (approved by the MHRA in early 2026) narrows that gap meaningfully, with trials reporting around 20.7% average loss at 72 weeks.

The table below summarises the key factual differences.

Both carry a Black Triangle (▼) designation, meaning the MHRA monitors them under additional surveillance. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance.

Side-effect profile: similar family, some nuance

The two medicines share a broadly similar side-effect profile because both slow gastric emptying. Nausea, loose stools, constipation, reflux and fatigue are the most commonly reported effects for both. These tend to cluster around the start of treatment or after a dose step up, and usually ease within a couple of weeks.

The main difference in tolerability data is one of degree rather than type, tirzepatide's GIP activity may influence how well the GI side effects are tolerated, though this varies person to person. The Mounjaro and Ozempic comparison page addresses GI tolerability in more detail if that's your main concern (noting that Ozempic is semaglutide licensed for type 2 diabetes, not weight loss, different product, same active ingredient as Wegovy).

One safety note that applies to both: the MHRA issued a Drug Safety Update in January 2026 flagging acute pancreatitis as an infrequent but potentially serious risk. Severe stomach pain spreading to the back, with or without vomiting, warrants urgent medical attention. Report any suspected side effects via the Yellow Card scheme.

For tirzepatide specifically, women taking oral contraceptives should use an additional non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because pill absorption may be reduced. This precaution doesn't have equivalent MHRA guidance for semaglutide. Discuss both medicines with your prescriber in the context of any other medicines you take.

Which one can you actually access, and through which route?

Both medicines are licensed for private prescription in the UK with the same BMI thresholds. Through a regulated pharmacy like nume, neither requires a GP referral or a wait. Your free consultation is reviewed by a GPhC-registered prescriber who decides which medicine, if either, fits your clinical picture.

On the NHS, the routes differ. Mounjaro is being phased in through GP practices under NICE TA1026, with eligibility criteria that are currently strict (broadly BMI ≥40 with multiple comorbidities) and expanding gradually. Wegovy remains primarily via specialist weight management services under NICE TA875, with commonly long waiting lists. The cost comparison page covers private pricing context in more detail.

If you're already on one medicine and thinking about switching, there are practical and clinical factors involved. The switching from Mounjaro to Wegovy page covers what that process looks like. The decision on which medicine to start, continue or switch to belongs with a prescriber who knows your situation. Speak to our prescribers and they'll work through it with you.