Mounjaro vs Wegovy: Which Weight Loss Injection is Right for You?
Mounjaro and Wegovy are the two most effective prescription weight loss injections currently available in the UK. Both work by influencing the hormones that regulate appetite and fullness — but they do so differently, deliver different average results in clinical trials, and may suit different patients.
This guide sets out the key differences in plain terms: how each treatment works, what the evidence shows, how they compare on dosing and side effects, and — most importantly — which might be the better starting point for you.
How each treatment works
How Mounjaro (tirzepatide) works – dual hormone action
Mounjaro contains tirzepatide, a molecule that activates two hormone receptors simultaneously: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). This dual agonist mechanism is what makes Mounjaro distinct from all other currently available weight loss injections. [1,3]
GLP-1 activation reduces appetite and slows gastric emptying, creating a feeling of fullness that persists longer after eating. GIP activation enhances the appetite-suppressing effect further and appears to improve how the body manages fat storage and energy use. The combination of both actions is thought to be the reason Mounjaro produces greater average weight loss than treatments that target GLP-1 alone. [1]
How Wegovy (semaglutide) works – GLP-1 receptor agonist
Both Wegovy injections and the Wegovy pillcontains semaglutide, a GLP-1 receptor agonist. It works by mimicking GLP-1, reducing appetite, increasing satiety and slowing the rate at which food leaves the stomach. Semaglutide has an established safety and efficacy profile, backed by the large STEP clinical trial programme and several years of real-world prescribing data across the UK and Europe [2,4].
Wegovy is also the same active ingredient as Ozempic — however Ozempic is licensed for type 2 diabetes rather than weight management and is prescribed through different clinical pathways [4].
Clinical trial results: what the evidence shows
Mounjaro and Wegovy have not yet been compared in a large published head-to-head randomised controlled trial for weight loss. The data below come from their respective Phase 3 programmes — SURMOUNT-1 for tirzepatide [1] and STEP 1 for semaglutide [2].
These trials used different populations and durations. The figures should be read as indicative rather than as a direct head-to-head comparison — though NICE has assessed both treatments independently in separate technology appraisals [3,4].
| Mounjaro (tirzepatide) | Wegovy (semaglutide) | |
| Key trial | SURMOUNT-1 [1] | STEP 1 [2] |
| Max studied dose | 15mg once weekly | 2.4mg once weekly |
| Trial duration | 72 weeks | 68 weeks |
| Avg. weight loss (max dose) | 20.9% [1] | 14.9% [2] |
| Placebo weight loss | 3.1% [1] | 2.4% [2] |
| % achieving ≥5% weight loss | 91% of participants [1] | 69% of participants [2] |
| % achieving ≥15% weight loss | 57% of participants [1] | 32% of participants [2] |
| % achieving ≥20% weight loss | 36% of participants [1] | 12% of participants [2] |
| NICE appraisal (England) | Recommended — TA1026 (Dec 2024) [3] | Recommended — TA875 (Mar 2023) [4] |
Across every milestone in the trial data, Mounjaro produces greater average weight loss than Wegovy [1,2]. However, a meaningful proportion of patients on Wegovy achieve significant results — 32% lost 15% or more of their body weight, and 12% lost 20% or more. Both treatments produce substantially better outcomes than placebo, and both are recommended by NICE for use in England for eligible patients.
These results are from separate clinical trials [1,2]. Individual results on either treatment will vary based on starting weight, dose reached, diet, activity levels and adherence.
Dosing: how each treatment is taken
Both Mounjaro and Wegovy are once-weekly subcutaneous injections administered under the skin — typically in the abdomen, thigh or upper arm. Both use pre-filled auto-injector pens. Both treatments are started at a low dose and gradually increased over several months to minimise side effects, particularly nausea in the early weeks [3,4].
Wegovy is also now available in tablet form, following MHRA approval on 11 June 2026— making it the first oral GLP-1 receptor agonist licensed for weight loss in the UK. Unlike the injection, the pill is taken once daily (rather than once weekly) and requires no refrigeration. The starting dose is 1.5mg, increasing through 4mg and 9mg up to a maximum of 25mg. Clinical trial data showed average weight loss of around 13.6% at the highest dose — slightly less than the injectable form, but a meaningful needle-free alternative for patients who prefer not to inject.
Your Bliss clinician will guide your dose escalation based on how you are tolerating treatment and the results you are achieving. Not every patient needs to reach the maximum dose — some achieve their goals at intermediate doses [3,4].
| Phase | Duration | Mounjaro dose [3] | Wegovy dose [4] |
| Starting dose | 4 weeks | 2.5mg/week | 0.25mg/week |
| Escalation | Months 2–6+ | 5mg → 7.5mg → 10mg → 12.5mg | 0.5mg → 1mg → 1.7mg |
| Maximum dose | Month 6+ | 15mg/week | 2.4mg/week |
Side effects: what to expect from each
Because both treatments activate GLP-1 receptors, their side effect profiles are broadly similar. The most commonly reported effects are gastrointestinal — nausea, diarrhoea, vomiting and constipation — and these are most likely to occur during dose escalation rather than at a stable maintenance dose [1,2].
| Side effect | Mounjaro [1,3] | Wegovy [2,4] |
| Nausea | Common — most likely during dose escalation | Common — most likely during dose escalation |
| Diarrhoea | Common | Common |
| Vomiting | Less common | Less common |
| Constipation | Less common | Less common |
| Injection site reactions | Mild, uncommon | Mild, uncommon |
| Pancreatitis (rare) | Rare — stop treatment and contact a clinician if severe abdominal pain occurs | Rare — stop treatment and contact a clinician if severe abdominal pain occurs |
For most patients, gastrointestinal side effects settle within a few weeks as the body adjusts to each new dose level. Eating smaller portions, avoiding high-fat meals and staying hydrated helps manage early effects [1,2].
Both weight loss treatments should not be taken by patients with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2 (MEN2). Neither is recommended during pregnancy or breastfeeding. A full clinical assessment at Bliss will screen for all relevant contraindications before any prescription is issued [3,4].
Which treatment is right for you?
There is no single answer that applies to everyone. The right treatment depends on your BMI, health history, how much weight you are looking to lose, and your personal preferences.
Mounjaro may be the better starting point if…
- You want the treatment with the strongest average weight loss results in clinical trials [1].
- You have a BMI of 35 or above and are looking for maximum weight loss potential.
- You have type 2 diabetes alongside obesity — tirzepatide is licensed and NICE-recommended for both [3].
- You have not previously tried a GLP-1 treatment and want to start with the most effective option currently available.
- Your clinician recommends it based on your individual health profile or clinical history.
Wegovy may be the better starting point if…
- You have previously taken Ozempic or another semaglutide-based treatment and tolerated it well — Wegovy uses the same active ingredient [2,4].
- You have had a positive initial response to semaglutide and want to continue on an established treatment.
- You would prefer to take a pill rather than an injection (the Wegovy pill)
- Your clinician recommends it based on your individual health profile or clinical history.
The best way to determine which treatment is right for you is through a clinical assessment. A Bliss clinician will review your health history, BMI, current medications and weight loss goals, and recommend the most appropriate option.
Your questions answered
Is Mounjaro better than Wegovy?
On average, Mounjaro produces greater weight loss than Wegovy in clinical trials. In SURMOUNT-1, participants on the 15mg dose lost an average of 20.9% of body weight at 72 weeks, compared with 14.9% in the STEP 1 trial for Wegovy at 68 weeks [1,2]. However, Wegovy remains a clinically effective treatment recommended by NICE [4]. The best choice depends on individual circumstances and should be guided by a clinician.
Are Mounjaro and Wegovy both approved in the UK?
Yes. Both are approved by the MHRA for use in the UK and have received positive technology appraisals from NICE for managing overweight and obesity in England. Mounjaro was appraised in NICE TA1026 (December 2024)[3] and Wegovy in NICE TA875 (March 2023)[4]. Both are available via private prescription at Bliss.
Can I switch from Wegovy to Mounjaro?
Yes, switching between GLP-1 treatments is possible and is sometimes recommended if you are not achieving your goals on your current treatment. Your Bliss clinician can advise on timing and dose escalation on a new treatment.
Which is more expensive — Mounjaro or Wegovy?
UK private pricing for both treatments varies by dose. As a general guide, both start from around £150–£200 per month at the lowest doses, rising at higher maintenance doses. Bliss will confirm current pricing for both treatments when you complete your assessment.
Are there any people who should not take either treatment?
Both treatments are contraindicated in patients with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome and are not recommended during pregnancy [3,4]. A full clinical assessment at Bliss identifies all contraindications before any prescription is issued.
References
[1] SURMOUNT-1 — Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity. New England Journal of Medicine (2022). Available from:
https://www.nejm.org/doi/10.1056/NEJMoa2206038[2] STEP 1 — Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine (2021). Available from:
https://www.nejm.org/doi/10.1056/NEJMoa2032183[3] NICE Technology Appraisal TA1026 — Tirzepatide for managing overweight and obesity (December 2024). Available from:
https://www.nice.org.uk/guidance/ta1026[4] NICE Technology Appraisal TA875 — Semaglutide for managing overweight and obesity (March 2023). Available from:
https://www.nice.org.uk/guidance/ta875
