Do GLP-1 Medicines Work? What the Evidence Shows (October 2026)

01 · The core answer

Do GLP-1 injections actually work?

Yes, for most people who take them as prescribed, though the size of the result differs from person to person. The clearest evidence for Mounjaro comes from the SURMOUNT-1 trial, where adults 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 trial average does not predict your own result. Some people lose more, some less, and the medicine only works while it is part of a wider plan.

It is also worth separating two questions. "Does it work?" asks whether the medicine produces meaningful weight loss on average. "Will it work for me?" depends on your dose, your tolerance and your habits. Our guide on whether these medicines work for everyone covers the second question in more depth.

02 · Your decision

Is this relevant to you, and what should you do next?

It is relevant if you are an adult looking for medically supervised weight management. Mounjaro is a once-weekly injectable prescription medicine. In the UK it is licensed for weight management in adults and for type 2 diabetes.¹ It is supplied only after a consultation reviewed by a GPhC-registered prescriber, who decides whether it is suitable and safe for you.

If you want to see how that works in practice, you can start with the Mounjaro pen prices and consultation. Your first 2.5mg starter pen costs £129.99 with code FIRST40 (standard price £169.99 per month). The consultation is free. If you would rather explore the wider picture first, our weight loss clinic page explains the options.

03 · How GLP-1 works

How GLP-1 medicines work

GLP-1 is a natural gut hormone, and these medicines activate its receptors to help regulate appetite, slow how quickly the stomach empties and improve the way the body responds to insulin. The effect is a combination of signals rather than a single switch.

Mounjaro adds a second target. 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. That last part matters. The medicine does not burn fat directly. It makes eating less easier to sustain, and that is where weight loss comes from.

This dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide (Wegovy/Ozempic).¹ Both groups are licensed, effective treatments. For a closer look at the whole family of medicines, see our explainer on how GLP-1 agonists work.

Different products also differ in how you take them. Mounjaro and Wegovy are once-weekly injections. Liraglutide, which is sold as Saxenda and in products such as Erly, is a daily injection supplied at 6mg/ml. Oral versions also exist, and we cover those separately in our piece on whether oral GLP-1 tablets work.

04 · Do they shrink the stomach?

Do GLP-1 medicines shrink your stomach?

No. They do not change the physical size of your stomach. What they change is how quickly it empties, and that is a different thing.

Slower emptying is one of the mechanisms described in the product information, alongside effects on appetite and insulin response.¹ Because food leaves the stomach more slowly, you tend to feel full sooner and stay satisfied for longer between meals.

That is why many people describe eating smaller portions without forcing it. It is also why large, rich meals can feel uncomfortable on treatment. Eating slowly and stopping when you feel satisfied tends to be more comfortable than eating to your old habits.

Slower stomach emptying is also the reason oral contraceptives can be absorbed less well. 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 that applies to you, tell your prescriber so they can advise.⁷

05 · What trials measured

What the trial evidence actually measured

The headline trial is SURMOUNT-1, which followed adults using tirzepatide alongside diet and exercise for 72 weeks. The same trial also reported how many people reached meaningful thresholds.

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 So the benefit was not confined to a small number of strong responders.

Two cautions apply. First, these are trial participants who were supported and followed closely, so real-world results may differ. Second, an average hides a spread. Some people lost far more than the average and some far less.

One further point on comparisons: SURMOUNT-5 is the first large head-to-head trial of tirzepatide and semaglutide for weight loss.¹⁵ Numbers from different trials should not be set side by side as if the medicines had been tested against each other.

06 · How fast and how much

How fast will you lose weight?

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.

Everyone starts on the same low dose. The starting dose is about letting your body adjust, not about weight loss itself. Our guide on how long GLP-1 takes to work breaks down the timeline further.

A sensible 3-month expectation on a standard titration is mid single digits as a percentage of body weight, with wide individual variation.² Slower early progress does not predict failure — the dose you respond to may still be ahead of you.

Treatment is also reviewed along the way. Treatment response is reviewed by your prescriber: 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.

For a worked example of the trial average, someone starting at 100 kg would lose about 21 kg at 20.9%.

07 · Dose ladder

The dose ladder and why higher is not always better

Everyone starts Mounjaro on 2.5mg once a week. 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

Many people settle at a maintenance dose of 5mg, 10mg or 15mg. Higher is not automatically better, and your prescriber will guide the right pace. Stepping up slowly is part of why side effects are manageable for most people.

Wegovy follows a similar idea with its own ladder: 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg once weekly, stepping up after at least four weeks and reaching the 2.4mg maintenance dose from week 17.¹⁸ You can see the Wegovy pen alongside Mounjaro if you are comparing.

08 · First days

What happens the first time you take it

Expect something fairly quiet. The 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, and both are normal. Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects and usually settle.

Early loss includes water weight, so the trend over weeks matters more than any single weigh-in.

The injection itself is simple. Mounjaro is injected under the skin once a week, on the same day each week, at any time of day, with or without food. Licensed sites are the stomach area (at least 5cm from the belly button), the front of the thigh, or the back of the upper arm if someone else gives the injection. Rotate the site with each dose.¹

There is no "best" time of day. What matters is the same day each week. Many people choose a weekend morning so early side effects fall on days off. Needles, a sharps bin and injection guidance are included with every order.

09 · The downsides

The honest downsides of GLP-1 treatment

The main downsides are digestive side effects, the need for ongoing commitment, and cost. The common early effects are mild nausea, altered appetite and tiredness in the days after each injection, and these usually settle.¹

The full list of possible side effects is in the patient information leaflet that comes with the pen.¹³ Read it before you start. If you notice a new symptom, speak to your prescriber, and you can report suspected side effects through the Yellow Card scheme.¹⁹

Other practical downsides are worth naming plainly:

• Results are gradual, so patience is part of the treatment.
• It works alongside a reduced-calorie diet and more activity. It does not replace them.
• Plateaus happen and can be frustrating.
• Treatment is private for many people, so cost is a real factor.
• Missed doses and travel need a little planning.

On cost, the standard monthly price of the 2.5mg pen is £169.99, checked 10 October 2026.

10 · Plateaus and staying on

Plateaus, and whether you stay on it forever

Plateaus are normal, and whether you continue long term is a decision to make with your prescriber. Weight loss on Mounjaro is not linear — plateaus of a few weeks are normal and expected as the body adapts to a lower weight.

Within the licence, the sensible responses are to review food intake honestly because portions drift back up, protect protein and activity, check the basics such as sleep and alcohol, and discuss with your prescriber whether a dose step-up is appropriate if you are not yet at maximum. A plateau is not a reason to double doses or add unlicensed products.

On stopping, the medicine itself clears gradually. Tirzepatide has a half-life of about five days, so after a final dose it takes roughly 4–5 weeks to clear the system — there is no way to speed this up, and nothing needs doing while it clears.¹ Mounjaro manages weight; it does not cure the biology behind it. 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.

There is no one-size-fits-all answer on how long to continue. Treatment is reviewed regularly, and our team carries out ongoing monthly reviews, dose titration and follow-up. Repeat orders re-check suitability, and there is no auto-shipping.

11 · Beyond the scales

Benefits beyond weight on the scales

In trials, weight loss on tirzepatide came with average improvements in waist size, blood pressure, cholesterol and blood-sugar measures, and most participants with prediabetes returned to normal blood-sugar levels.²

These are trial-average secondary outcomes, not promises. They are, however, why prescribers talk about weight management as health management rather than appearance.

If you want to test the claims in this area, our page on whether GLP-1 actually works goes through the evidence and the caveats in a similar way.

12 · Safest option and NHS

Which GLP-1 is safest, and what about the NHS?

There is no single "safest" GLP-1. The right choice depends on your health history, other medicines and what you tolerate, which is exactly what a prescriber reviews. Licensed, prescribed treatment from a registered pharmacy is the safe route. Unregulated sellers, and products offered without a consultation or prescription, are not.

You can check any UK pharmacy on the GPhC register. Ours is registered with the GPhC under number 9012878.

On NHS access, Mounjaro is available on the NHS, but access is being phased in over several years. 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).⁴⁶ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.

Ask your GP what is available in your area.

13 · Beware of gimmicks

Gimmicks that borrow the GLP-1 name

Some products use the GLP-1 label without containing a licensed medicine. Patches, stickers and low-cost supplements promising dramatic weight-loss claims are not the same as prescribed tirzepatide or semaglutide. Our page on whether GLP-1 stickers work looks at one such product.

Be cautious of anyone offering to supply prescription injections without a consultation or prescription. The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme for reporting problems.⁷ Our Mounjaro is supplied through official LillyDirect supply, direct from Eli Lilly's UK distribution, as genuine KwikPens.

14 · 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