Should I Try Weight Loss Injections? What the Evidence Says

What large trials actually found, and why it matters for your decision

Two landmark clinical trials anchor most of the evidence you will read about. The STEP 1 trial, published in the New England Journal of Medicine, followed adults with obesity over 68 weeks and found that those taking semaglutide 2.4mg lost an average of around 15% of their body weight, compared with roughly 2.4% on placebo. SURMOUNT-1, which studied tirzepatide over 72 weeks, reported average reductions of around 20–21% at the highest dose among 2,539 participants, figures that prompted NICE to recommend tirzepatide for eligible adults in England.

Those numbers deserve honest framing. They are averages from controlled trials with lifestyle support built in, so individual results vary. Some people lose less; a smaller number lose substantially more. There is also a common misconception worth clearing up gently: these injections do not act on fat directly. They activate gut-hormone receptors involved in appetite and blood-sugar regulation, making it easier to eat less rather than melting anything away. That distinction matters because it explains why food choices and activity still make a real difference to outcomes.

A head-to-head comparison (the SURMOUNT-5 trial, also published in the New England Journal of Medicine in 2025) found that tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks. If you are also considering cost-effective alternatives, our page on weight loss injections in generic form explains what options may be available and how they relate to the branded treatments covered in these trials.

Who these medicines are licensed for in the UK

The UK licence for both Mounjaro and Wegovy covers adults with a BMI of 30 or above, or a BMI of 27 or above if at least one weight-related health condition is present, for example, high blood pressure, high cholesterol, obstructive sleep apnoea, prediabetes or type 2 diabetes. For some South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, UK guidance applies BMI thresholds that are 2.5 kg/m² lower, reflecting differences in metabolic risk. You can read more about this on our weight loss injection overview page.

Eligibility on paper does not mean a prescriber will approve treatment automatically. Your full medical history matters: certain gastrointestinal conditions, a personal or family history of medullary thyroid carcinoma or MEN2, or current pregnancy are among the circumstances where these medicines are not appropriate. Neither injection is licensed for people under 18, and both should be stopped if you are trying to conceive, pregnant or breastfeeding. Our page on who should not take weight loss injections covers these exclusions in detail.

The honest answer to whether you qualify is one a prescriber can give after reviewing your specific situation, not a checklist result.

Side effects and what tolerability looks like day to day

Nausea is the side effect most people ask about first, and it is the one most likely to occur, particularly in the early weeks or after a dose increase. Vomiting, loose stools, constipation, reflux and low-grade fatigue are also reported. The good news is that for most people these effects are mild to moderate and tend to ease as the body adjusts, usually within days to a couple of weeks at each dose level. Starting at a low tolerability dose (2.5mg for tirzepatide; 0.25mg for semaglutide) before gradually increasing is specifically designed to reduce this burden.

More serious effects are uncommon but worth knowing. The MHRA has highlighted acute pancreatitis as a recognised though infrequent risk: if you develop severe, persistent stomach pain that reaches the back, with or without vomiting, seek urgent medical help and mention you are on a GLP-1 medicine. Gallbladder symptoms and signs of allergic reaction also warrant prompt attention. You can report suspected side effects at any time via the MHRA Yellow Card scheme. Full side-effect information is in the Patient Information Leaflet supplied with your medication.

If you are weighing up the injection route against the newer oral option, our weight loss treatments overview sets out what is currently available and how the approaches compare.

How long treatment typically lasts and what happens next

This is a question our prescribers hear regularly, and the answer is: it depends. NICE's appraisal of tirzepatide (TA1026) recommends reviewing continued use if less than 5% of body weight is lost after six months at the highest tolerated dose, a sensible clinical checkpoint that shifts focus from the medicine to measurable results. For semaglutide, NICE guidance specifies a maximum of two years within a specialist service, though private prescribing arrangements differ.

Evidence from extension studies consistently shows that weight tends to return when treatment stops, because the underlying biology that drives appetite does not change permanently. That does not mean injections are a lifelong commitment for everyone, but it is a conversation to have with your prescriber before you start, so expectations are grounded in evidence. Our page on how long to take weight loss injections explores this further, as does the related question of how long you should stay on treatment.

If you would like to explore whether treatment is suitable for you, our prescribers are available for a free same-day consultation. There is no obligation, and no prescription is issued unless treatment is clinically appropriate for your specific situation. Speak to our prescribers to get started.