- What they are: Mounjaro = tirzepatide (GIP + GLP-1); Wegovy = semaglutide (GLP-1). Same class, different molecules.
- Efficacy: In the SURMOUNT-5 head-to-head, tirzepatide beat semaglutide for average weight loss (−20.2% vs −13.7%).
- NHS route: Wegovy via specialist weight-management services only (NICE TA875, max 2 years); Mounjaro for obesity via a phased NHS rollout (NICE TA1026).
- Price: Both are usually paid for privately. Mounjaro's UK list prices rose sharply from 1 September 2025.
- Availability: Both have largely recovered from the 2022–24 GLP-1 squeeze; occasional strength-level gaps still happen.
- Bottom line: No self-selecting. Suitability, side-effect tolerance and access all matter — decide with a prescriber.
Same class, different drugs
Both belong to the incretin family of weight-management and diabetes medicines, and both are given as a once-weekly injection under the skin. The pharmacology differs:
- Wegovy (semaglutide) activates one gut-hormone receptor, GLP-1. The same molecule is sold as Ozempic (licensed for type 2 diabetes) and Rybelsus (a semaglutide tablet for diabetes). For weight management in the UK, the licensed brand is Wegovy.
- Mounjaro (tirzepatide) activates two receptors, GIP and GLP-1. In the UK it is licensed for both weight management and type 2 diabetes.
Both work by reducing appetite, slowing stomach emptying and improving blood-sugar control. Neither is a substitute for the diet, activity and follow-up that a prescriber wraps around them. This guide deliberately does not give doses — tirzepatide and semaglutide must be started, titrated and monitored by your prescriber.
Efficacy: what the trials actually show
The most reliable comparison comes from SURMOUNT-5, the first large randomised trial to test the two drugs directly in adults with obesity but without type 2 diabetes (751 participants, 72 weeks). Tirzepatide produced greater average weight loss:
| Trial (design) | Drug & dose | Mean weight change | Comparator |
|---|---|---|---|
| SURMOUNT-5 (head-to-head, 72 wks) | Tirzepatide (max tolerated) | −20.2% | Semaglutide 2.4 mg: −13.7% |
| SURMOUNT-1 (tirzepatide vs placebo, 72 wks) | Tirzepatide 15 mg | −20.9% | Placebo: −3.1% |
| SURMOUNT-1 | Tirzepatide 10 mg | −19.5% | Placebo: −3.1% |
| STEP-1 (semaglutide vs placebo, 68 wks) | Semaglutide 2.4 mg | −14.9% | Placebo: −2.4% |
In SURMOUNT-5 that difference meant an average loss of about 22.8 kg on tirzepatide versus 15.0 kg on semaglutide. Sources: the head-to-head trial (Aronne et al., N Engl J Med 2025; DOI 10.1056/NEJMoa2416394), SURMOUNT-1 (NEJM 2022, PMID 35658024) and STEP-1 (NEJM 2021, PMID 33567185).
Licensing & NHS access: the routes differ
This is where the two medicines diverge most in the UK, and it often matters more than the efficacy gap.
Wegovy (semaglutide) on the NHS
NICE recommends Wegovy for weight management in TA875, but only within a specialist weight-management service (a GP cannot start it directly), for a maximum of two years. Eligibility needs at least one weight-related health condition plus a BMI of 35 or more (or 30–34.9 while meeting specialist-referral criteria), with thresholds set 2.5 points lower for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds. Treatment may be stopped if less than 5% of weight is lost after six months. Specialist-service capacity is limited, so waits can be long.
Mounjaro (tirzepatide) on the NHS
For obesity, NICE recommends tirzepatide in TA1026 (published 23 December 2024). Because so many people are eligible, NHS England is rolling access out in phases under its interim commissioning guidance, starting with the highest clinical need and widening over years — so availability depends heavily on where you live and which phase your area is in. Separately, for type 2 diabetes, Mounjaro is available on the NHS as a later-line option under a different appraisal, TA924.
Price: what changed, and what to check
Neither medicine is cheap privately, and pricing shifted in 2025. The clearest verified change is Mounjaro's: Eli Lilly raised its UK list prices (what it charges pharmacies, not the final price you pay) from 1 September 2025:
| Mounjaro strength | Old list price | New list price (from 1 Sep 2025) |
|---|---|---|
| 2.5 mg | £92 | £133 |
| 5 mg | £92 | £180 |
| 7.5 mg | £107 | £255 |
| 10 mg | £107 | £255 |
| 12.5 mg | £122 | £330 |
| 15 mg | £122 | £330 |
Source: Community Pharmacy England, which negotiated matching NHS reimbursement prices so pharmacies would not dispense at a loss. Private clinics add consultation and dispensing fees on top of the list price, so the amount you actually pay is higher and varies between providers and by dose. Private Wegovy pricing is set by individual clinics and also varies widely; we do not publish a single figure because there isn't a reliable, official one to cite. The safe approach with either drug is to ask your provider for the current total monthly cost at your dose before committing, and to be wary of any price that looks too cheap to be genuine.
Availability in 2026
Surging worldwide demand in 2022–24 caused patchy supply across the whole GLP-1 class, and in the UK the DHSC restricted GLP-1 medicines to people with type 2 diabetes during the worst of it. By 2026 both tirzepatide and semaglutide supply is reported to have largely recovered, though — as with any high-demand medicine — a particular pharmacy can still be briefly out of a specific strength. You can follow official UK supply signals on our live pages for Mounjaro, tirzepatide, Wegovy and semaglutide. If you rely on either, reorder in good time and confirm stock rather than waiting until your last pen.
Side effects & safety: broadly similar
Both drugs share a similar safety profile. The most common effects are gut-related — nausea, vomiting, diarrhoea and constipation — which usually ease over the first weeks. More serious risks to know about for both include:
- Pancreatitis: seek urgent help for severe, persistent tummy pain, especially with vomiting.
- Gallbladder problems (gallstones, inflammation) are recognised, uncommon effects.
- Pregnancy and breastfeeding: not to be used; stop in good time before trying to conceive and ask your prescriber how long to leave. The MHRA has warned that tirzepatide may make the oral contraceptive pill less reliable in women who are overweight — discuss contraception with your prescriber.
- Muscle loss: rapid weight loss on any GLP-1 medicine includes some lean mass — see our guide on GLP-1 medicines and muscle loss.
In SURMOUNT-5, gastrointestinal side effects leading people to stop treatment were somewhat less common with tirzepatide (2.7%) than semaglutide (5.6%), but tolerance is individual. Never start, stop, split or change a dose without advice from your prescriber or pharmacist. For plain-English overviews, see the NHS pages on tirzepatide and semaglutide.
So which is "better"?
On average weight loss, the trial evidence favours Mounjaro (tirzepatide). But "better in a trial" does not settle what is right for an individual. The medicine you can actually access on the NHS in your area, the one you tolerate, your other health conditions, and what you can afford privately all feed into the decision — and only a prescriber can weigh those against your history. Do not self-select or switch between these medicines on your own. Book a conversation with your GP, pharmacist or a specialist weight-management service, and if you are ever acutely unwell (for example severe abdominal pain), contact NHS 111 or emergency services.
Related reading
Mounjaro (tirzepatide) in the UK
Availability, the 2025 price rises and the phased NHS rollout.
Wegovy & Ozempic (semaglutide)
The difference between the brands, and how to access them.
Do they cause muscle loss?
What the trials show about lean mass, and how to protect muscle.
Orforglipron: an oral GLP-1
Where the first oral small-molecule GLP-1 stands for UK availability.
Get told the moment supply changes
MediWatch checks official DHSC and NHS data daily and alerts you if your medication is affected.
Search shortages free →Official & primary sources: SURMOUNT-5 head-to-head (NEJM 2025) · SURMOUNT-1 (NEJM 2022) · STEP-1 (NEJM 2021) · NICE TA1026 (tirzepatide, obesity) · NICE TA875 (semaglutide, weight management) · NICE TA924 (tirzepatide, diabetes) · Community Pharmacy England price update · MHRA: fake weight-loss pens · NHS: tirzepatide · NHS: semaglutide
MediWatch is not medical advice. Always follow your prescription label and ask a pharmacist, GP, specialist, NHS 111, or emergency services if you are unsure or unwell. Data checked daily against official sources.