What is the success rate of weight loss injections in the UK?

What do the clinical trials show about average weight loss?

This is a question our prescribers hear most weeks, and it deserves a precise answer rather than a vague reassurance. The headline figures come from two major trial programmes. In SURMOUNT-1, adults without diabetes who received tirzepatide 15mg lost an average of around 20–21% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. That is roughly 22–23 kg for someone starting at 110 kg. The STEP 1 trial tested semaglutide at its 2.4mg maintenance dose over 68 weeks and found an average loss of approximately 15%, still a clinically meaningful result by any measure. When the two medicines were compared directly in the SURMOUNT-5 head-to-head trial (72 weeks, 751 adults with obesity), tirzepatide produced greater average weight reduction than semaglutide 2.4mg. A newer, higher-dose semaglutide option at 7.2mg is narrowing that gap in more recent data. These figures are averages. Some participants lost considerably more; others lost less. The trials do not predict what any one person will experience, but they do confirm that these medicines produce results well beyond what diet and exercise alone typically achieve in people living with obesity.

For context on what the 3 weight loss injections are and how each one works, that page covers the options in detail.

Why do results vary so much from person to person?

The trial averages mask a wide spread of individual outcomes, and understanding why matters if you are thinking about starting treatment. Dose reached is probably the single biggest variable. Both tirzepatide and semaglutide are titrated gradually over months, and people who reach and tolerate the higher maintenance doses tend to lose more weight. Those who stop at a lower dose due to side effects will see a smaller reduction on average. Starting BMI also matters: people with a higher starting weight often lose a larger absolute number of kilograms, even if the percentage figure is similar. Adherence to lifestyle changes alongside the injection makes a real difference too, the trials were conducted with structured dietary support, not the injection alone. Finally, individual biology plays a part. GLP-1 and GIP receptors respond differently between people, and genetic factors that influence appetite regulation, metabolism and gut motility are still being studied. None of this means the medicines do not work; it means that weight loss injection treatment works best when it is clinically supervised and supported, rather than treated as a standalone fix.

What does 'success' actually mean in this context?

Percentage of body weight lost is the metric most trials use, but clinicians assess success differently. NICE's appraisal of tirzepatide (TA1026) includes a practical decision point: if someone has not lost at least 5% of their starting weight after six months at the highest tolerated dose, continuing is reviewed. That 5% threshold is not a failure mark, it is a checkpoint to ensure the medicine is doing something useful for that individual. In practice, most people who tolerate the dose escalation do reach that threshold and go well beyond it. Separately, weight loss of even 5–10% of body weight carries measurable health benefits: reduced blood pressure, improved blood sugar control and lower cardiovascular risk. So success is not an all-or-nothing figure. A 10% reduction that a person sustains long-term may deliver more health value than a 20% reduction that reverses when treatment stops. That is why ongoing clinical review, not just the starting prescription, shapes real-world outcomes. You can read about how people describe their experience with weight loss injections alongside the clinical data.

Are there safety considerations that affect how long people stay on treatment?

Success rates in the real world are also shaped by tolerability. The most common reason people reduce or stop treatment early is gastrointestinal side effects: nausea, loose stools, constipation, indigestion and fatigue. These are most noticeable when starting or after a dose increase and tend to ease within one to two weeks for most people. Staying at a dose for longer before moving up, eating smaller meals and keeping well hydrated all help. More rarely, some people experience symptoms that need prompt attention. In January 2026, the MHRA issued a Drug Safety Update highlighting acute pancreatitis as a known but uncommon risk with GLP-1 medicines: severe, persistent stomach pain that may radiate to the back warrants urgent medical assessment. Knowing what to watch for does not make the medicines less appropriate for most people; it makes treatment safer. Longer-term, the data from trials lasting up to 72 weeks is encouraging, but these are relatively new medicines and post-marketing surveillance continues. Understanding the real risks of weight loss injections (rather than the sensationalised version) helps you go into treatment with clear expectations. If you are weighing up which injection might suit you, a comparison of how the options perform covers the evidence side by side. For questions about cost alongside everything else that goes into a treatment decision, our treatment page sets out what is included. Speak to our prescribers if you would like a clinical assessment of whether these medicines are right for you specifically.