GLP-1 Explained: What It Is and What GLP-1 Medicines Do (October 2026)

01 · The basics

What is GLP-1 and what does it do?

GLP-1 is a hormone made in your gut and released when you eat. It does three jobs: it prompts the pancreas to release insulin when blood sugar rises, it slows how fast the stomach empties, and it tells the brain you are full. The result is steadier blood sugar and less hunger between meals.

The natural hormone is broken down very quickly in the body. GLP-1 medicines are engineered versions that last far longer, which is why they can work from a single weekly or daily injection.

02 · Your next step

Is a GLP-1 medicine relevant to me?

It may be if you are an adult living with obesity, or with overweight plus a weight-related health condition, and lifestyle changes alone have not been enough. These medicines support a reduced-calorie diet and more physical activity; they do not replace them. Some are also licensed for type 2 diabetes and used under the care of a clinician.

If you want to explore it, our weight loss clinic starts with a free online consultation reviewed by a GPhC-registered prescriber. If you already know you want Mounjaro, the Mounjaro pen starts at £169.99 per month for the 2.5mg starting dose (prices checked 10 October 2026).

Related guides: Ozempil glp-1 and Wat is glp-1 and Trapezoid glp 1 and Ampk and glp-1.

03 · Mounjaro and GLP-1

Is Mounjaro a GLP-1 medicine?

Mounjaro is a GLP-1 medicine, but it is not only that. Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1.¹ Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin. In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.

Mounjaro is the brand name for tirzepatide, 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

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.

04 · UK GLP-1 medicines

Which GLP-1 medicines are used in the UK?

Several licensed options exist, and they differ in active ingredient, how often you inject and what they are licensed for.

Wegovy's titration steps up after at least four weeks and reaches the 2.4mg maintenance dose from week 17.¹⁸ Tablet versions of GLP-1 medicines have also been licensed; see current Wegovy tablet details on our Wegovy page and the product page for prices.

Other manufacturers are also active in this field, as in our note on Boehringer Ingelheim's GLP-1 work.

05 · Safety and legality

Are GLP-1 medicines legal and safe in the UK?

Yes, licensed GLP-1 medicines are legal in the UK, but only as prescription-only medicines. Mounjaro (tirzepatide) is supplied only after an online consultation reviewed by a GPhC-registered prescriber, and used alongside a reduced-calorie diet and increased physical activity.¹ Anyone offering to supply without a consultation or prescription is outside the regulated route.

Like any medicine, they have side effects. The most common are digestive: nausea, diarrhoea, vomiting, constipation and abdominal discomfort, affecting more than 1 in 10 people. Side effects are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks. Tell your prescriber about anything new or troubling.¹³ You can report suspected side effects through the Yellow Card scheme.¹⁹ 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.

Two interactions are worth knowing. Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed, so ask your prescriber about contraception.⁷ 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, your doses may need adjusting because of the risk of low blood sugar.¹³

06 · Combinations to avoid

What should you not combine with GLP-1 treatment?

Do not combine GLP-1 medicines with one another. Mounjaro must not be combined with other GLP-1 medicines, unlicensed 'research' peptides (such as retatrutide, which has no UK licence), or compounded copies. These combinations have no safety data and no regulated supply chain. If something promises faster results outside a licensed pathway, that is the warning sign.

Supplements marketed as natural alternatives are not GLP-1 medicines. They are low-cost, have no clinical oversight and do not have the evidence behind the licensed treatments. Dramatic weight-loss claims on such products should be treated with caution.

07 · 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

08 · Stopping and NHS access

NHS access and what happens when you stop

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).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. NHS access to Wegovy runs through specialist weight-management services; ask your GP.

Tirzepatide has a half-life that means that 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 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.