Guides

How GLP-1 Medicines Work

How GLP-1 receptor agonists act on appetite, digestion and blood sugar, why doses are increased gradually, what happens when treatment stops, and key safety points.

JoinGRI Editorial Team · 16 October 2026 · 4 min read

Acting on the body's GLP-1 receptors

After you eat, your gut releases a hormone called GLP-1, which helps control blood sugar, appetite and digestion. Natural GLP-1 is broken down within minutes. Our article What does the GLP-1 hormone do? explains the natural hormone.

GLP-1 receptor agonists are medicines that bind to the same receptors as natural GLP-1. "Agonist" means they switch the receptor on. They are designed to last much longer than the natural hormone, so their effects continue between doses. Some newer medicines act on two receptors: GLP-1 and another gut hormone receptor called GIP.

Most are given as injections, and some are available as tablets for type 2 diabetes. For an overview that also covers other types of weight loss medicine, see how weight loss medication works.

Effects on appetite

These medicines act on areas of the brain involved in hunger and fullness. Many people find they feel full sooner, feel less hungry between meals and eat smaller portions. How strongly this happens varies from person to person.

Effects on digestion

They slow gastric emptying, the rate at which food leaves the stomach. This contributes to feeling full for longer, and it is also linked to some of the common digestive side effects, such as nausea. Slower stomach emptying also matters before surgery or procedures under sedation (see the safety section below).

Effects on blood sugar

Like natural GLP-1, these medicines help the pancreas release insulin when blood sugar is raised and reduce the release of glucagon. This is why they were first developed for type 2 diabetes. Low blood sugar is a possible side effect, so prescribers may need to review other diabetes medicines someone takes.

Why doses are increased gradually

Prescribers usually start at a low dose and increase it in steps over several weeks or months. Starting low gives the body time to adjust and can reduce digestive side effects. The dose and timing are always set by the prescriber, who may keep someone on a lower dose if side effects are troublesome. Doses shouldn't be changed without advice.

What these medicines can't do

• They don't work in the same way for everyone, and results vary.
• They aren't a replacement for changes to food and activity. In the UK they are licensed for use alongside a reduced-calorie diet and increased physical activity.
• They aren't suitable for everyone. Some medical conditions, other medicines, pregnancy and breastfeeding can mean they aren't appropriate. Our eligibility page explains what clinicians usually consider.
• The MHRA notes that using these medicines is not a quick way to lose weight.

What happens when treatment stops

Evidence considered by NICE shows that weight tends to be regained after these medicines are stopped, and that some of the benefits seen during treatment can be lost over time. On the NHS, NICE recommends semaglutide for weight management for a maximum of 2 years, within a specialist weight management service.

This is one reason building lasting habits around food, activity and sleep matters during treatment, not just afterwards. Anyone thinking about stopping should talk to their prescriber first.

Safety considerations

Common side effects

Digestive side effects, such as feeling sick, being sick, diarrhoea and constipation, are the most commonly reported, particularly when starting or increasing the dose. They often ease over time.

Serious side effects

Less commonly, serious problems can occur. The MHRA notes rare reports of pancreatitis: severe stomach pain that spreads to the back and doesn't go away needs urgent medical help. Severe dehydration from sickness or diarrhoea can also be serious. Our guide to weight loss medication side effects covers what to look out for.

Pregnancy and contraception

The MHRA advises that GLP-1 medicines should not be taken during pregnancy or just before trying to get pregnant, and that anyone who could become pregnant should take steps to avoid pregnancy while using them. Depending on the medicine, it may need to be stopped at least 1 to 2 months before trying to conceive.

Tirzepatide may make oral contraceptive pills less effective in some people. The NHS advises using an additional barrier method, such as condoms, for 4 weeks after starting tirzepatide and for 4 weeks after each dose increase. A pharmacist or prescriber can advise on contraception.

Surgery and sedation

Because these medicines slow stomach emptying, the MHRA advises telling the healthcare team that you take one before any operation or procedure under general anaesthetic or deep sedation. Don't stop treatment without discussing it with your doctor first.

Reporting side effects

You can report suspected side effects to the MHRA through the Yellow Card scheme.

This article is for general education only. It isn’t medical advice and doesn’t replace a conversation with a qualified healthcare professional about your own circumstances.