GLP-1 Definition: What It Means and How It Works (October 2026)

01 · The definition

What is the medical definition of GLP-1?

Medically, GLP-1 is an incretin hormone: a signal released by the gut after eating that tells the body how to handle the food. Its name comes from glucagon-like peptide-1.

Three effects matter most. It helps regulate appetite, slows how quickly the stomach empties, and improves the way the body responds to insulin.¹ In practice, most people on a GLP-1 medicine feel fuller sooner and less hungry between meals.

02 · Hormone or drug?

GLP-1 hormone versus GLP-1 medicine

GLP-1 is not the same as Ozempic: Ozempic is a branded medicine (semaglutide) that acts on the hormone's receptor, and it is licensed for type 2 diabetes only.

03 · Is this for you?

Is a GLP-1 medicine relevant to you?

It may be if you are an adult with weight-related health concerns. NICE recommends Mounjaro for adults with a BMI of at least 35 plus at least one weight-related condition, and NHS access is being phased in over several years.⁴ Many people who meet the licensed criteria choose regulated private treatment. You can start with our weight loss clinic, or look at the Mounjaro pen and its current prices, where a free consultation is reviewed by a GPhC-registered prescriber.

Related guides: Agonisti glp 1 and Trapezoid glp 1 and Wat is glp-1.

Related guide: Definition of appetite suppressant.

04 · How GLP-1 medicines work

How GLP-1 medicines work

Natural GLP-1 breaks down very quickly, so the medicines are built to last far longer in the body. Mounjaro is once weekly: tirzepatide has a half-life of about five days, and after a final dose it takes roughly 4–5 weeks to clear the system.¹

Tirzepatide activates two natural gut-hormone receptors at once, GIP and GLP-1. This dual action distinguishes it from single-action medicines such as semaglutide (Wegovy/Ozempic).¹ Our Mounjaro definition page covers the product itself.

05 · What results to expect

What results can GLP-1 medicines deliver?

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.

06 · Safety and who should avoid

Are GLP-1 medicines safe, and who should avoid them?

GLP-1 medicines are licensed prescription treatments, and suitability is decided case by case by a prescriber. 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. Always share your full medical history and medicine list during consultation. Mounjaro is prescription-only and supplied after an online consultation reviewed by a GPhC-registered prescriber.

Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed.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 Report any suspected side effect through the Yellow Card scheme.¹⁹ 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.

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