Which injectable is best for weight loss in the UK?

What the head-to-head trial found, and what it means for you

For years, comparing Mounjaro and Wegovy meant looking at separate trials run in different populations at different times. That changed with SURMOUNT-5, published in the New England Journal of Medicine in 2025. It was a direct, randomised comparison: 751 adults with obesity and no diabetes, followed for 72 weeks. Tirzepatide at its highest tolerated dose produced significantly greater average weight reduction than semaglutide 2.4 mg. That is the most reliable evidence available, because it eliminates the design differences that make separate trial results awkward to compare.

SURMOUNT-1, the tirzepatide-only phase 3 trial, had already reported around 20–21% average body-weight reduction at 15 mg over 72 weeks. The STEP 1 trial for semaglutide 2.4 mg, published in the New England Journal of Medicine, reported roughly 15% average loss over 68 weeks. These figures matter, but they are population averages from tightly controlled settings, individual results spread widely around them.

It is also worth knowing that a newer, higher semaglutide dose exists. The MHRA approved a dedicated single-dose 7.2 mg Wegovy pen in April 2026, and trials at that dose reported around 20.7% average weight loss, which narrows the gap between the two medicines considerably. Whether that dose is suitable for a given person is something a prescriber works out during assessment. If you are trying to weigh up your options, our page on which is the best weight loss injection looks at how these medicines compare across different circumstances. That is precisely why these medicines require a prescription.

How the mechanisms behind these injectables differ in practice

Wegovy works on a single pathway: it mimics GLP-1, a gut hormone released after eating that signals fullness to the brain, slows how quickly food leaves the stomach, and influences insulin release. Mounjaro does all of that and adds a second pathway, it also activates GIP receptors. GIP is another gut hormone involved in appetite regulation and, potentially, fat metabolism. Activating both may explain why tirzepatide tends to produce larger average losses at comparable durations; the honest answer is that the exact reasons are still being studied.

In practical terms, both medicines are given by once-weekly subcutaneous injection, and our guide to which injection site is best for weight loss explains the differences between the abdomen, thigh, and upper arm, including how to rotate sites correctly each time. Both start at a low tolerability dose and are titrated upward by the prescriber over several months. Neither is a quick fix; the titration schedule exists to let your body adjust and to keep side effects manageable.

The side-effect profile is broadly similar across both: nausea, loose stools, constipation, indigestion and fatigue are the most commonly reported, usually most noticeable at the start or after a dose increase, then settling for most people. If you want a detailed breakdown by medicine, our guide to weight-loss injections covers both side-effect profiles alongside practical storage and injection advice. One symptom to take seriously on either treatment is severe stomach pain that reaches through to the back, seek medical help promptly if that happens, and report unexpected reactions through the MHRA's Yellow Card scheme.

Eligibility: who these injectables are licensed for

Both medicines are licensed for adults. Mounjaro's licence covers a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Wegovy's injection licence covers the same BMI thresholds. For some ethnic backgrounds, UK guidance applies thresholds that are 2.5 kg/m² lower, your prescriber applies the correct threshold for you.

NICE has appraised both medicines for NHS use, with specific criteria attached to each. NICE's recommendation for tirzepatide (TA1026) requires a BMI of 35 or above plus at least one weight-related comorbidity for NHS access, with phased rollout criteria. NHS waiting lists for both treatments are significant in many areas. Private treatment through a regulated pharmacy removes the waiting list but not the clinical assessment, a prescriber still reviews your case before anything is dispensed.

If you are comparing costs alongside clinical fit, our treatment overview page sets out what private treatment includes. For a more personal steer on which injectable might suit your situation, the page on which weight-loss injection is best for me walks through the factors that shape that clinical decision.

What the prescriber considers when choosing between them

Evidence and licensing set the boundaries; the prescriber fills in the detail. They will look at your current medications, tirzepatide can temporarily reduce how well oral contraceptive pills are absorbed, so women taking the pill are advised to use an additional non-oral method for the first four weeks of treatment and for four weeks after each dose increase. HRT formulation may also come up, since NHS England advises considering transdermal patches or gels during tirzepatide treatment for the same absorption reason.

Existing conditions matter too. A history of certain thyroid conditions, pancreatitis or specific GI disorders may affect which treatment is suitable or whether either is appropriate right now. Pregnancy, breastfeeding, and trying to conceive are all reasons neither medicine is recommended; the same applies to anyone under 18.

None of this can be worked out by a website. It requires a real conversation, or, in our case, a thorough written consultation reviewed by a named prescriber the same day. A question our prescribers hear most weeks is something like: "I've done my research online; can you just tell me which one to pick?" The honest answer is that even with all the trial data in front of you, the right choice only emerges once someone looks at your whole picture. That is what the consultation is for. If you want to explore your options and get a clinical view on which weight loss injectable suits you, you can check your eligibility with our prescribers at no charge. Further reading on which injectable peptide tends to work best for weight loss may also help before you start the process.