How Do GLP-1s Work? (October 2026)

01 · The short version

What GLP-1 medicines actually do

GLP-1 medicines act on the gut-hormone system that tells your body when you have eaten. The natural hormone helps regulate appetite, slows how quickly the stomach empties and improves the way the body responds to insulin.¹

In practice, most people feel fuller sooner and less hungry between meals.¹ It makes a reduced-calorie diet and increased physical activity easier to stick to, which is how the treatment is licensed to be used.¹

02 · What next

Is this relevant to you, and what next?

This matters if you are considering a prescription weight-management treatment and want to understand what you would be taking. In the UK, Mounjaro is licensed for weight management in adults and for type 2 diabetes.¹ Whether it suits you is a decision for a prescriber, not a web page.

If you are ready to look at treatment, you can see the Mounjaro pen, prices and free consultation, or browse our wider weight loss clinic. Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber. Individual results vary, and this page is information, not a substitute for medical advice.

03 · Hormone mechanism

The gut-hormone mechanism

Mounjaro activates the GLP-1 receptor, together with the GIP receptor, and this is how it helps regulate appetite.

Three effects matter for weight. They help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹

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

04 · GLP-1 versus GIP

Single-action versus dual-action medicines

Not every GLP-1 medicine works identically. Semaglutide (Wegovy, Ozempic) is a single-action GLP-1 medicine. Tirzepatide, the active ingredient in Mounjaro, is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1.¹

That dual action is what distinguishes Mounjaro from single-action GLP-1 medicines.¹

Liraglutide, found in Saxenda and similar products, is also a GLP-1 medicine, but it is a daily injection, unlike once-weekly Mounjaro and Wegovy. The Wegovy ladder runs 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg, stepping up after 4 weeks on each of 0.25mg, 0.5mg, 1mg and 1.7mg, so the 2.4mg maintenance dose starts from week 17.¹⁸ Mounjaro comes in 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg strengths.¹

05 · What results look like

What results look like in trials

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, and individual results vary. Treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.² Trial averages do not predict your own result.

In the same trials, weight loss 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 secondary outcomes, not promises. They are why prescribers talk about weight management as health management rather than appearance.

06 · Pace and plateaus

How quickly do GLP-1s work?

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.²

Some people notice appetite change and early loss in week one. Others feel little until a higher dose, and both are normal. Early loss includes water weight, so the trend over weeks matters more than any single weigh-in.

After a few months, a sensible expectation is modest progress, with wide individual variation. In SURMOUNT-1, average weight loss built month by month, with the full averages measured at 72 weeks. Slower early progress does not predict failure, because the dose you respond to may still be ahead of you.²

Weight loss on Mounjaro is not linear — plateaus of a few weeks are normal and expected as the body adapts to a lower weight. If one happens, review food intake honestly, protect protein and activity, and check sleep and alcohol. Then ask your prescriber whether a dose step-up is appropriate. A plateau is not a reason to double doses or add unlicensed products.¹¹

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

07 · Downsides and limits

The downsides and limits

The commonest early effects are mild nausea, altered appetite and tiredness in the first days after each injection, and they usually settle.¹³ If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention. You can also report any suspected side effect through the Yellow Card scheme.¹⁹

There are limits beyond side effects. These medicines are not appetite-suppressant pills, and they do not replace diet and activity. Our guide to how appetite suppressants differ explains the contrast. Products marketed as natural or drink-based alternatives do not have the same evidence or oversight; check a Mounjaro drink claim carefully.

Treatment also does not end instantly. Tirzepatide has a half-life of about five days, so after a final dose it takes roughly four to five weeks to clear the system. There is no way to speed this up, and nothing needs doing while it clears.¹

08 · Access in the UK

Getting GLP-1 treatment in the UK

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. Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.⁴

Private treatment through EverydayMeds, a GPhC-registered pharmacy (9012878), starts with a free online consultation reviewed by a GPhC-registered prescriber. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full. Delivery is free next-day tracked in temperature-controlled packaging, with same-day dispatch when ordered by 3pm. Mounjaro comes through official LillyDirect supply, and ongoing monthly reviews are included.

Wegovy is the other injectable option; see the Wegovy pen.

09 · 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