Weight Loss Injections UK Study: What the Research Shows (October 2026)

01 · The core answer

What do the main weight loss injection studies show?

The main trial behind Mounjaro is SURMOUNT-1, which tested tirzepatide in adults with obesity. In that trial of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages — individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

The pattern to notice is that average loss rose with dose. It also means the lifestyle-only group lost far less than the treated groups, which is why the trial is used to support licensing. If you want a plain-English walk-through of the headline trial, our Mounjaro study explainer goes through it step by step.

02 · Your decision

Is this research relevant to me?

It is relevant if you are an adult considering a licensed weight-management medicine and want to know what is realistic. Mounjaro is licensed in the UK for weight management in adults, so the trial population is the group the licence describes.¹ A trial is not a promise, though. Treat the average as a range of possible outcomes, with some people above it and some below.

The practical decision most readers face is whether to pursue NHS access, private treatment, or neither. If you decide private treatment is right, the next step is a free consultation with a prescriber who reviews your health history. Our weight loss clinic page explains how that works, and the Mounjaro pen starting dose costs £169.99 per month at standard price. New customers save £40 on their first order with code FIRST40, and £25 off every later order with code DOSE25.

Related guide: Hse weight loss injections.

03 · How the jab works

How do weight loss injections work?

Weight loss injections work by acting on gut-hormone signals that control appetite and digestion. Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹

In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹ That is why the studies always pair the medicine with diet and activity. The injection changes appetite; it does not do the eating and moving for you.

Mounjaro is the brand name for tirzepatide, a once-weekly injectable prescription medicine made by Eli Lilly. It comes as a pre-filled multi-dose pen (KwikPen) in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹ Semaglutide (Wegovy) acts on a single hormone pathway, while liraglutide is a daily injection supplied at 6mg/ml. The trials for each medicine are separate, so their headline numbers should not be set side by side as if they were a direct comparison.

04 · Reading the trial results

How to read the trial results properly

The most useful way to read the results is to look at both the average and the spread. The average tells you the middle; the proportions tell you how many people reached meaningful milestones. At 72 weeks, 85% of participants on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5% of their body weight.² Half of participants on 10mg and 57% on 15mg lost at least 20% of their body weight.²

Percentages can feel abstract, so here is a worked example. For someone starting at 100 kg, 20.9% is about 21 kg. That is the straight arithmetic of the trial average at the top dose, not a forecast for any individual and not a timetable.

Three cautions apply to any weight loss jab study:

• Averages hide variation. Some people lose more than the average and some lose less. Trial averages do not predict your own result.
• Trials are controlled. Participants get regular support. Real-life results depend on diet, activity, tolerability and follow-up.
• Dose matters, but higher is not automatically better. Many people settle at a maintenance dose of 5mg, 10mg or 15mg, and your prescriber guides the pace.¹

If you are weighing a newer headline against this one, our piece on a new study on weight loss drugs shows how to judge whether it changes anything for you.

05 · Realistic pace

How fast do results arrive in studies and in practice?

Results arrive gradually, and studies measure them over many months rather than days. Weight loss on Mounjaro is gradual by design. The first weeks on 2.5mg are an adjustment phase and losses are usually modest; most measurable progress builds from 5mg onward, month by month. Trials measured results at 72 weeks — think in months, not days, and judge progress by trend, not individual weigh-ins.²

That fits the dosing schedule. Everyone starts on 2.5mg once a week. This starting dose is about letting your body adjust, not about weight loss itself. After at least 4 weeks the dose increases to 5mg, and can then step up by 2.5mg at a minimum of 4-week intervals (7.5mg, 10mg, 12.5mg) up to a maximum of 15mg weekly, depending on how you respond and tolerate it.¹

The first weeks vary a lot between people. The 2.5mg starting weeks are about adjustment, and experiences differ widely: some people notice appetite change and early loss in week one, others feel little until a higher dose — both are normal. Early loss includes water weight, so the trend over weeks matters more than any single weigh-in.¹

Your response is also reviewed. If you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, your prescriber will discuss whether continuing or stopping is right for you.¹

06 · UK usage and NHS access

What does the UK usage study say, and what about the NHS?

A UCL study published in BMC Medicine estimated how many adults in Great Britain have used weight loss injections such as Wegovy and Mounjaro, and how many more are interested. It reported that use was more common in women than men, was concentrated in middle-aged adults, and was overwhelmingly private rather than through the NHS.

The private pattern makes sense given how access is being phased in. Mounjaro is available on the NHS, but access is being phased in over several years.⁶ NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.

For the UK, ask your GP about NHS routes first, and read the NHS guidance on the medicine itself.⁵

A usage study also raises a safety point. When most people buy privately, the quality of the supplier matters. Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber. Anyone offering to supply without a consultation or prescription is not operating safely. You can check any pharmacy on the GPhC register; ours is GPhC 9012878.¹²

07 · What studies cannot tell you

What can studies not tell you?

Studies cannot tell you how a medicine will suit your own body, your own medicines or your own life. They give averages from a defined group over a defined period.

Keeping weight off. Maintaining weight is a long-term task. In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued treatment lost a further 5.5% on average. Stopping should always be planned with your prescriber, with diet and activity habits ready to carry the load.¹⁴

Mood and mental health. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment. You can also report suspected side effects through the MHRA Yellow Card scheme.¹⁹

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

Smell and other odd symptoms. Body odour changes are not a listed side effect in the UK patient leaflet. Discuss any new symptom with your prescriber.

Related conditions. Research into other conditions is separate from the weight trials. Our page on the Mounjaro and PCOS study explains what that area does and does not show.

08 · Natural alternatives and myths

Are there natural or cheaper alternatives to weight loss jabs?

No supplement has been shown to match a licensed medicine. Dosing is not standardised, and these products have not been tested in the trials above.

Marketing for such products often makes dramatic weight-loss claims. Treat those as a warning sign. A licensed medicine has a product licence, a patient leaflet, a known side-effect profile and a prescriber who monitors you. A supplement has none of those.

The same applies to "tricks". Nothing replaces the combination the trials tested: a medicine, a reduced-calorie diet and more physical activity. If you want a lifestyle-first approach with support, that is a legitimate choice, and your GP or pharmacist can help you plan it.

If you do decide on a licensed route, you can register for weight loss injections online and start with a free consultation.

09 · Using the jab safely

What does safe use look like once you start?

Safe use means following the licensed schedule exactly. Mounjaro is injected under the skin (subcutaneously) once a week, on the same day each week, at any time of day, with or without food. Rotate the site with each dose, and each KwikPen contains four weekly doses.¹

If you miss a dose, take it as soon as you can within 4 days (96 hours) of the scheduled time, then carry on with your usual day. If more than 4 days have passed, skip the missed dose entirely and take the next one on your normal day — never take two doses within 3 days of each other.¹

Keep pens in a fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded.¹

Our prescribing team provides ongoing monthly reviews and side-effect support, and repeat orders re-check suitability. Mounjaro is used alongside a reduced-calorie diet and increased physical activity. Individual results vary, and this page is information, not a substitute for medical advice.

10 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹