Which weight loss injection is best for me?

How do Mounjaro and Wegovy actually work differently?

Wegovy contains semaglutide, a GLP-1 receptor agonist. It mimics a gut hormone that signals fullness to the brain, slows how quickly the stomach empties, and reduces the drive to eat. It has been used in diabetes treatment for years and its mechanisms are well understood.

Mounjaro contains tirzepatide, which does the same job but also activates a second receptor, the GIP pathway. GIP is another gut hormone involved in blood-sugar regulation and fat storage. Activating both receptors appears to produce a stronger appetite-suppressing effect than either alone. The NHS medicines page for tirzepatide describes it as the first medicine of its kind to combine these two pathways for weight management.

In practical terms, both slow gastric emptying (which is why you feel full on smaller portions) and both reduce calorie intake without requiring conscious restriction. The difference is one of degree and mechanism, not of category. Neither is a shortcut; both work alongside a reduced-calorie diet and increased physical activity. A weight management overview can help set those expectations before you reach a consultation.

One thing worth knowing early: the starting dose for each is deliberately low. For Mounjaro, treatment begins at 2.5mg, that first pen exists to let your body adjust, not to produce immediate results. Wegovy follows a similar logic. Prescribers titrate slowly to reduce the likelihood of nausea or other GI symptoms in the early weeks.

What does the trial evidence say about results?

Results in clinical trials are averages across large groups, and individual responses vary. That caveat matters. With it firmly in place, here is what the data show.

In the SURMOUNT-1 trial, involving 2,539 adults with obesity, participants taking the highest dose of tirzepatide lost an average of around 20–21% of body weight over 72 weeks. Semaglutide 2.4mg (Wegovy) produced around 15% average weight loss over 68 weeks in the STEP 1 trial, as published in the New England Journal of Medicine. Those figures come from different trials with different participant groups, so direct comparison is difficult.

The most meaningful head-to-head evidence comes from SURMOUNT-5, published in the New England Journal of Medicine in 2025. In that open-label trial of 751 adults with obesity and no diabetes, tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks. NICE's appraisal of tirzepatide (TA1026) also notes that indirect comparisons favour tirzepatide.

A newer option worth knowing about is the Wegovy 7.2mg dose, approved by the MHRA in April 2026. Trials reported around 20.7% average weight loss at that higher dose, narrowing the gap with tirzepatide considerably. Which dose is appropriate for you, and when, is a clinical decision, and if you want to explore the broader field our guide to which injectable is best for weight loss covers how these options compare in more detail. You can read a more detailed comparison of these two injections for a deeper look at the numbers.

Who can be prescribed each injection, and does eligibility differ?

Both medicines share a broadly similar licensed eligibility threshold: adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol, obstructive sleep apnoea, or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance.

Beyond the raw BMI figure, your prescriber looks at contraindications. A personal or family history of medullary thyroid carcinoma or MEN2 syndrome, active pancreatitis, or certain gastrointestinal conditions will affect which medicine (if any) is appropriate. Pregnancy, breastfeeding, and trying to conceive rule out both. Neither is licensed for under-18s.

There is one practical eligibility note specific to Mounjaro: women taking oral contraceptives should add a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide's effect on gastric emptying may reduce how reliably the pill is absorbed. Semaglutide does not carry the same evidence for pill absorption concerns, though your prescriber will discuss contraception whichever medicine you use.

If cost is part of your thinking, the cost of weight loss injections in the UK varies by dose and provider and is worth understanding before you start. And if you are wondering about injection technique and which sites to use, that is a useful practical question to have answered alongside the clinical one.

So which injection is actually right for me?

Honestly, this question cannot be answered in a single article, and any source claiming otherwise is selling something. What the evidence does clearly support is that both injections are effective, both are clinically supervised, and tirzepatide currently shows stronger average results in trials. For many people that makes Mounjaro the first clinical conversation. For others, semaglutide's longer track record, or the new 7.2mg option, makes it the better fit.

A prescriber weighs your full picture: BMI, any conditions, any medicines you already take, your preferences about injections versus other formats, and your history with previous weight-management attempts. There is no algorithm that does this well, and our guide to what weight loss injection is best for me walks through the key factors a prescriber considers if you want to think it through before your consultation. Our prescribers at nume are GPhC-registered Independent Prescribers who review every consultation personally, one of them reads your answers the same day you submit them, not a bot working through a queue.

Many people submit their consultation form before noon and find a decision has been made by the time they are thinking about the afternoon. That is the point of same-day clinical review. If you would like to talk through which injection might suit you, speaking to our prescribers is the right next step, it is free, there is no obligation, and the consultation itself is a clinical service, not a sales process.