Updated regularly — Last updated June 2025
Mounjaro vs Wegovy: a thorough, balanced comparison for the UK
Both Mounjaro and Wegovy have strong clinical evidence behind them. Both are real, effective options. Understanding the meaningful differences — and the important nuances — helps you have a better-informed conversation with your prescriber.
Important context for these numbers
SURMOUNT-1 and STEP-1 were different clinical trials, conducted in different patient populations, with different durations. They cannot be compared as directly as they might appear. The figures above are averages — individual responses to both medications vary substantially. A head-to-head comparison (SURPASS-CVOT) exists in the cardiovascular outcomes context but not specifically for weight management in directly comparable populations. Your individual response may differ significantly from either average.
The key difference: mechanism
The most important clinical difference between Mounjaro and Wegovy is how they work in the body.
Wegovy (semaglutide) is a GLP-1 receptor agonist. It mimics a single gut hormone — GLP-1 — that signals fullness to the brain, slows gastric emptying, and stimulates insulin release after eating.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. It activates both GLP-1 and GIP receptors simultaneously. GIP (glucose-dependent insulinotropic polypeptide) is a second gut hormone that plays a role in insulin secretion, fat metabolism, and appetite. The combination appears to produce a synergistic effect that results in greater average weight loss than GLP-1 activation alone.
This is the most likely explanation for why Mounjaro consistently shows greater average weight loss than semaglutide in comparative data. It is not simply a "stronger" dose of the same thing — it is a genuinely different pharmacological mechanism.
Clinical evidence compared
| Factor | Mounjaro (SURMOUNT-1) | Wegovy (STEP-1) |
|---|---|---|
| Trial duration | 72 weeks | 68 weeks |
| Top dose tested | 15mg once weekly | 2.4mg once weekly |
| Average weight loss (top dose) | 20.9% | 15.2% |
| Average kg lost (top dose) | ~21.8 kg | ~15.3 kg |
| % achieving ≥20% loss | 57% (at 15mg) | ~33% |
| % achieving ≥15% loss | 71% (at 15mg) | 51% |
| Placebo (lifestyle only) | 3.1% | 2.4% |
| Published in | NEJM, 2022 (Jastreboff et al.) | NEJM, 2021 (Wilding et al.) |
Caution: these were separate trials with different patient populations, different durations, and different study designs. They are not directly comparable — but they are the best available evidence. Individual results vary substantially within both trials.
What this evidence means in practice
Mounjaro produces greater average weight loss in its landmark trial. However: (1) both produce clinically meaningful weight loss far beyond placebo; (2) individual responses vary greatly within each treatment; (3) the "better" medication is the one that works for you as an individual and that your prescriber considers most appropriate. Some people respond extremely well to Wegovy; some achieve outstanding results with Mounjaro. The averages don't determine your outcome.
Side effects compared
Both medications share a broadly similar side effect profile due to their shared GLP-1 mechanism. There are some differences in reported incidence.
| Side effect | Mounjaro | Wegovy |
|---|---|---|
| Nausea | ~29–33% at 5mg | ~44% overall |
| Diarrhoea | ~17–23% | ~30% |
| Constipation | ~11% | ~24% |
| Vomiting | ~9–13% | ~24% |
| Sulphur burps | Very common (real-world) | Also reported, possibly less |
| Hair shedding | Widely reported — telogen effluvium | Similar — same mechanism |
| Serious (pancreatitis, gallbladder) | Rare — listed in prescribing info | Rare — listed in prescribing info |
Some clinical reports suggest Wegovy may have a higher incidence of certain GI side effects, while Mounjaro users more frequently report sulphur burps. Both patterns are plausible given their mechanisms, but real-world differences in individual experience are substantial. Full Mounjaro side effects guide → · Full Wegovy side effects guide →
Dosing compared
| Phase | Mounjaro | Wegovy |
|---|---|---|
| Initiation | 2.5mg × 4 weeks | 0.25mg × 4 weeks |
| Therapeutic levels | 5mg, 7.5mg, 10mg, 12.5mg | 0.5mg, 1.0mg, 1.7mg |
| Maintenance dose | 15mg (max) | 2.4mg (max) |
| Total titration | ~20 weeks to max dose | ~16 weeks to max dose |
| Frequency | Once weekly | Once weekly |
Injection devices
Mounjaro uses the KwikPen — a single-press auto-injector. Press firmly against the skin, press the button, hold for 10 seconds. Each pen contains one pre-set dose. Colour-coded by dose level.
Wegovy uses the FlexTouch pen — a dial-and-press injector. Select the dose on the dial, press and hold the injection button until the dose counter returns to zero. Multiple dose settings possible on some devices.
Both are suitable for self-injection. Most people who are initially nervous about injecting report that the first injection was much easier than anticipated. Full injection guides: Mounjaro → | Wegovy →
NHS approval and access
Both medications received NICE approval in 2023 — Wegovy under TA875, Mounjaro under TA1026. Access through NHS GPs remains very limited for both, with most provision through Tier 3 specialist services (subject to eligibility and ICB funding). The NHS pilot also covers both medications in participating areas. Full NHS access guide →
UK private prescription costs compared
| Dose phase | Mounjaro (approx.) | Wegovy (approx.) |
|---|---|---|
| Starter | ~£149–£165/month | ~£149–£165/month |
| Mid-therapeutic | ~£185–£219/month | ~£185–£215/month |
| Maximum dose | ~£215–£275/month | ~£199–£239/month |
Approximate figures, June 2025. Prices vary by pharmacy. SlimBee earns no commission from any pharmacy. Always verify current pricing directly.
Who might prefer each medication?
The most important thing
This decision should be made with your prescriber based on your complete medical history, current medications, comorbidities, and individual risk factors. Both are excellent medications with strong evidence. Neither SlimBee nor anyone else on the internet is a substitute for a proper clinical assessment by a GMC-registered prescriber who knows your full picture.
Can you switch between Mounjaro and Wegovy?
Yes — switching between these medications is done in clinical practice. You cannot simply swap one for the other at "equivalent" doses, as the dosing scales are completely different and the mechanisms differ. Switching requires: stopping one medication, having a fresh medical assessment, obtaining a new prescription, and starting the new medication from its lowest initiation dose and titrating up properly.
Common reasons for switching include suboptimal response to one medication, side effects that don't improve, changes in NHS availability, or patient preference after discussion with their prescriber. Never switch without proper medical supervision.
A note on accessing either medication privately
Our guide to UK pharmacy and prescriber types covers everything you need to know about choosing a provider safely — what to check, what red flags look like, and how the different models work.
Frequently asked questions
Mounjaro produces greater average weight loss in its landmark clinical trial (20.9% vs 15.2% in separate trials). Its dual mechanism appears to explain this difference. However: these were different trials with different populations; individual response to both varies enormously; "better" is the medication that works for you and is appropriate for your clinical circumstances. Some people do extremely well on Wegovy; some achieve outstanding results with Mounjaro. Your prescriber should help you decide.
Yes, but it requires a full new medical assessment and a fresh prescription. You cannot simply swap at an equivalent dose — the dosing scales are completely different. You would need to start the new medication from its lowest initiation dose and titrate up properly. Speak to your prescriber about the process and any washout period considerations.
Both are incretin-based therapies — they work by mimicking gut hormones. Wegovy is a GLP-1 receptor agonist. Mounjaro is a dual GIP/GLP-1 receptor agonist. They share a GLP-1 mechanism, which is why their side effect profiles are similar, but Mounjaro's additional GIP receptor activity makes it pharmacologically distinct rather than just a modified version of semaglutide.
Trial data suggests Wegovy may have somewhat higher reported rates of certain GI side effects (nausea, diarrhoea, vomiting, constipation). Mounjaro users may report sulphur burps more frequently. However, both profiles are broadly similar, individual experience varies enormously, and you cannot predict your personal experience from trial averages. Many people tolerate both medications very well.
Medical disclaimer: For informational purposes only. Always consult a GMC-registered prescriber before starting or changing medication. SlimBee has no commercial pharmacy ties. Last reviewed June 2025. References: Jastreboff AM et al. NEJM 2022 (SURMOUNT-1); Wilding JPH et al. NEJM 2021 (STEP-1); NICE TA875; NICE TA875. Full disclaimer →