Is GLP-1 Good for Weight Loss? An Honest UK Answer (October 2026)

01 · The core answer

So, is GLP-1 any good for weight loss?

Yes, for the right person it is good, and the trial evidence is strong. In the SURMOUNT-1 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 That is the honest answer to "how good is GLP-1 for weight loss": very good on average, unpredictable for any one person.

02 · Your decision

Should I go on GLP-1 for weight loss?

That depends on your health, your goals and whether you can commit to the lifestyle side. A GLP-1 medicine is a prescription treatment, not a shortcut, so the useful questions are practical ones.

• Do you meet the licensed criteria? A prescriber checks this against your health history.
• Can you pair it with food and activity changes? The trial results depended on both.
• Are you ready for side effects and monthly follow-up? Good care includes regular reviews.
• Can you plan for the long term? Decide with your prescriber how you will approach stopping before you start.

If those answers are yes, a free consultation with a GPhC-registered prescriber is the next step. You can see how treatment works at our weight loss clinic, or look at the Mounjaro pen, where the starting 2.5mg pen costs £169.99 per month at the standard price (prices checked 10 October 2026).

Related guide: Is mounjaro a good weight loss drug.

03 · How GLP-1 works

How GLP-1 medicines work

GLP-1 medicines copy natural gut hormones that help regulate appetite and digestion. Tirzepatide, the active ingredient in Mounjaro, activates GIP and GLP-1 receptors 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 dual action distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide, which is the active ingredient in Wegovy and Ozempic.¹

Mounjaro is a once-weekly injectable prescription medicine made by Eli Lilly. In the UK it is licensed for weight management in adults and for type 2 diabetes. It comes as a pre-filled multi-dose pen (KwikPen) in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹ Our guide to whether Mounjaro is any good covers it in more detail.

04 · What the trial shows

What the SURMOUNT-1 results mean

The trial figures show that higher doses produced larger average losses, but also that many people responded well at lower doses. At 72 weeks, 85% of participants on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5% of their body weight. Half of participants on 10mg and 57% on 15mg lost at least 20% of their body weight.²

Put as a worked example, for someone starting at 100 kg, 20.9% is about 21 kg. That is straight arithmetic on a trial average, not a promise, and it should not be read as a timetable. Some people lose less and some lose more.

This also answers "how fast do people lose weight on GLP-1?". The trial measured results over 72 weeks, and speed varies from person to person. Treatment also starts low and steps up gradually, so the early weeks are about building tolerance. Our page on Mounjaro injection results goes through what to expect.

Please do not compare these numbers with results from other trials of other medicines as if they were head to head. Different trials recruit different people and measure different things.

05 · Risks and side effects

Is GLP-1 worth the risk?

For most suitable adults the balance is favourable, but "worth it" is a judgement you make with a prescriber who knows your history. All effective medicines have side effects, and GLP-1 medicines are no exception. The most commonly reported effects involve digestion, and the full list sits in the patient information leaflet.¹³

Read the leaflet, tell your prescriber about any new or worrying symptoms, and report suspected side effects through the Yellow Card scheme.¹⁹ Raising a problem early helps you stay safe on treatment.

The biggest side effect for most people is digestive discomfort, particularly while the dose is rising. Common effects such as nausea, diarrhoea, vomiting, constipation and abdominal discomfort are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks as the body adjusts. If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.

Who cannot take GLP-1? Mounjaro is not suitable for everyone. You should not take it if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive. Your prescriber will take extra care, or may recommend against treatment, if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy, or gallbladder problems. Pregnancy, certain medical histories and some other medicines can all matter. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly. Contraception is one example: Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed. The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ If you take insulin or sulfonylureas, doses may need adjusting because of the risk of low blood sugar.

06 · Belly fat and your face

Does GLP-1 reduce belly fat, and what does it do to your face?

Overall weight reduction is what the trials measured. Where you lose it depends on you.

The same goes for "GLP-1 face". Losing weight quickly by any method can change how the face looks, and that is a general feature of weight loss rather than something we can pin on a specific medicine. If you notice changes that worry you, speak to your prescriber, and report any suspected side effect through the Yellow Card scheme.¹⁹

Eating enough protein and staying active are sensible habits while on treatment, and our guide to foods that suit GLP-1 treatment offers practical ideas.

07 · Mounjaro and GLP-1

Is GLP-1 the same as Mounjaro?

Not exactly. GLP-1 is the name of a gut hormone and of the class of medicines that mimic it. Mounjaro is a specific brand whose active ingredient, tirzepatide, activates GIP and GLP-1 receptors together, which is why it is often described as dual-action.¹

Wegovy is a semaglutide product, a single-action GLP-1 medicine. Liraglutide, sold as Saxenda, is a daily injection supplied at a concentration of 6mg per millilitre, while Mounjaro and Wegovy are once weekly. You can read about the Wegovy pen as well, and a visual summary sits in our GLP-1 infographic.

Wegovy starts low and steps up after 4 weeks on each of 0.25mg, 0.5mg, 1mg and 1.7mg, and the 2.4mg maintenance dose starts from week 17.

08 · 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