What Is GLP-1? The Hormone, the Medicines and How They Work

GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It helps control blood sugar and appetite. GLP-1 medicines copy it, and in the UK they are licensed to treat type 2 diabetes, manage weight, or both. This guide explains the hormone, the medicines, how they differ and what the MHRA says about using them safely.

What Is GLP-1?

A natural hormone released by the gut after eating that helps control blood sugar and appetite.

GLP-1, or glucagon-like peptide-1, is a natural hormone released by the gut after eating. The product information for semaglutide describes it as a physiological hormone with several actions on blood sugar, appetite and the cardiovascular system, with its effects on blood sugar and appetite working through GLP-1 receptors in the pancreas and the brain.

GLP-1 medicines act on the same receptors but are designed to last much longer in the body. Mounjaro, for example, is described in its product information as long-acting, with a half-life of about 5 days, which is why it is injected once a week.

What Are GLP-1 Medicines?

Medicines that copy GLP-1; licensed for diabetes, weight management or both, depending on the product.

GLP-1 receptor agonists, often shortened to GLP-1s or GLP-1RAs, are medicines that copy GLP-1. The MHRA describes them as medicines that "help people feel fuller by mimicking a natural hormone released after eating", and notes that some newer medicines, like Mounjaro, also act on a second hormone.

The MHRA's own table of which are licensed for what:

• Semaglutide as Wegovy: licensed for weight management, not for diabetes.

• Semaglutide as Ozempic and Rybelsus: licensed for type 2 diabetes, not for weight management.

• Tirzepatide (Mounjaro): licensed for both. It acts on GIP as well as GLP-1.

• Liraglutide: depends on the brand; for example Saxenda is licensed for weight management.

Orforglipron (Foundayo), a daily tablet, is also licensed for both weight management and type 2 diabetes, according to its product information.

You may see them called "weight loss injections" or "skinny jabs", but, as the MHRA points out, not all of them are authorised for weight loss.

How Do GLP-1 Medicines Work?

They reduce appetite, lower high blood sugar and slow stomach emptying.

According to their product information, GLP-1 medicines:

• Reduce appetite: GLP-1 receptors are found in brain areas that regulate appetite and food intake, so people feel fuller, less hungry and eat less.

• Lower blood sugar when it is high: they help the pancreas release insulin and lower glucagon, a hormone that raises blood sugar, in a glucose-dependent way.

• Slow stomach emptying: this slows the rise in blood sugar after meals.

For more detail on individual medicines, see what is Wegovy, what is Ozempic and how Mounjaro works.

Injections and Tablets

As injections or tablets; no licensed GLP-1 medicine is a patch, drop or oral liquid.

Licensed GLP-1 medicines in the UK come in two forms:

• Injections: weekly pens such as Wegovy, Ozempic and Mounjaro, and daily liraglutide pens such as Saxenda.

• Tablets: Rybelsus and Wegovy tablets (semaglutide) and Foundayo (orforglipron), taken daily.

None of the licensed GLP-1 medicines comes as a patch, drops or an oral liquid. Products sold that way online are not licensed medicines; see GLP-1 patches, drops and oral solutions.

Who Can Use GLP-1 Medicines?

Prescription-only; the weight-management ones are for adults with a BMI of 30+, or 27+ with a condition.

GLP-1 medicines are prescription-only. The weight-management ones are licensed for adults with a body mass index (BMI) of 30 kg/m² or more, or 27 kg/m² or more with a weight-related condition, alongside a reduced-calorie diet and more physical activity. The MHRA notes it has not assessed their safety and effectiveness outside their licensed use, for example for weight loss in people who are not overweight or obese.

The MHRA advises only taking GLP-1 medicines prescribed to you by a healthcare professional, and talking to a prescriber before switching between them, because they vary in strength, how they are used and what they are licensed for.

Side Effects and Risks

Mostly stomach-related; rare pancreatitis and, with semaglutide, very rare NAION.

The MHRA says the most common side effects are stomach-related, such as nausea, vomiting and diarrhoea, and that most are mild to moderate or short-lived. Some can lead to severe dehydration needing hospital care.

• Pancreatitis: infrequent, but serious or fatal cases have been reported rarely. Seek urgent medical help for severe stomach pain that spreads to the back and does not go away.

• Eyesight: semaglutide has, in very rare reports, been linked to NAION, an eye condition. Contact a doctor urgently for a sudden change in eyesight.

• Surgery: tell the team you take a GLP-1 medicine if you are due to have an operation.

• Gallstones are a common side effect listed in the product information of these medicines.

• Diabetes: with insulin or a sulfonylurea, the risk of low blood sugar is higher, and those doses may be lowered. In people with pre-existing diabetic retinopathy, a rapid improvement in blood sugar has been linked to a temporary worsening, so they should be monitored closely.

Report suspected side effects to the MHRA Yellow Card scheme. See Wegovy side effects and Mounjaro side effects.

Pregnancy, Breastfeeding and Contraception

Not in pregnancy or breastfeeding; stop semaglutide 2 months and tirzepatide 1 month before trying.

The MHRA advises that GLP-1 medicines should not be taken during pregnancy, while trying to get pregnant or while breastfeeding, and that anyone who could become pregnant should use contraception. It sets how long before trying to get pregnant they should be stopped:

• Semaglutide (Wegovy, Ozempic, Rybelsus): at least 2 months.

• Tirzepatide (Mounjaro): at least 1 month.

• Liraglutide: just before, as it leaves the body much more quickly.

• Other GLP-1 medicines: follow the advice in their product information and from your prescriber.

If you take Mounjaro and the contraceptive pill, add a barrier method such as condoms for 4 weeks after starting and after each dose increase, or switch to a non-oral method.

Frequently Asked Questions