GLP-1 Basics . FAQ

GLP-1 FAQ: What It Is, How the Medicines Work, and What to Expect

October 21, 2026 · 11 min read
GLP-1 FAQ: What It Is, How the Medicines Work, and What to Expect
TL;DR

GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It signals fullness to the brain and helps release insulin when blood sugar is high, but your body breaks it down in about a minute and a half. GLP-1 medicines such as semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) act on the same receptor and last about a week, so they can be taken once weekly. In the EU, Wegovy and Mounjaro are authorised for weight management in adults with a BMI of 30 or more, or 27 or more with a weight-related condition, alongside diet and activity. Nausea, diarrhoea, vomiting and constipation are very common, mostly while the dose is being raised. Weight tends to come back after stopping. Because people eat less on these medicines, they also tend to eat less fibre.

GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It tells the brain you are full and helps release insulin. GLP-1 medicines such as semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) act on the same receptor and last about a week, so they can be taken once weekly.

Each medicine is covered in more depth in our guide to GLP-1 medications in Europe. For fibre specifically, see the complete guide to fiber and GLP-1 medications. None of this replaces advice from the person who prescribes your medicine.

What GLP-1 is

What is GLP-1?

GLP-1 stands for glucagon-like peptide-1. It is a hormone made mainly by L-cells in the lower small intestine and colon, and by some neurons in the brain. It is released when you eat and increases fullness and helps regulate blood sugar.1

Is GLP-1 a hormone or a drug?

Both words are used. GLP-1 is the hormone your body makes. “GLP-1” is also everyday shorthand for the medicines built to act like it. Their proper name is GLP-1 receptor agonists: they bind to and switch on the same receptor as the natural hormone.2

What does GLP-1 do in the body?

Levels are low when you are fasting and rise after a meal.1 The European Wegovy label describes GLP-1 as “a physiological regulator of appetite and calorie intake”, with receptors in several parts of the brain involved in appetite. It also helps lower blood sugar by stimulating insulin and lowering glucagon when blood glucose is high.2

Why does natural GLP-1 wear off so fast?

An enzyme called DPP-4 breaks it down. A 2024 review puts its half-life at about a minute and a half.1 Semaglutide is built to resist this and has a half-life of about a week, which is why it can be injected once a week.2 Our post on oatzempic explains why that gap matters for food-based “GLP-1 boosters”.

The medicines

What are GLP-1 medications?

They are medicines that activate the GLP-1 receptor. Semaglutide shares 94% of its sequence with human GLP-1.2 Tirzepatide is a dual agonist that acts on the GIP and GLP-1 receptors.3

Which GLP-1 medicines are available in Europe?

The main ones authorised in the EU:

  • semaglutide (Ozempic): weekly injection for type 2 diabetes.
  • semaglutide (Wegovy): weekly injection for weight management. A daily 25 mg Wegovy tablet was authorised by the European Commission on 14 July 2026.4
  • semaglutide tablet (Rybelsus): daily, for type 2 diabetes.
  • tirzepatide (Mounjaro): weekly injection for type 2 diabetes and for weight management.3
  • liraglutide (Victoza, Saxenda): older daily injections, for type 2 diabetes and weight management.

Orforglipron (Foundayo), a newer daily pill, is approved in the US and the UK. In the EU, the EMA’s medicines committee (CHMP) recommended it for approval on 15 October 2026, and the European Commission has not yet decided.5 Our GLP-1 medications guide covers each one in more detail.

What is the difference between Ozempic and Wegovy?

They are the same molecule, semaglutide. Ozempic is authorised for type 2 diabetes. Wegovy is authorised for weight management, with a maintenance dose of 2.4 mg once weekly.2 Because the doses differ, the two labels report different side-effect rates: nausea affected 17% and 19.9% of people at the Ozempic doses of 0.5 mg and 1 mg,6 and 43.9% at the Wegovy dose.2

Is Mounjaro a GLP-1?

Partly. Tirzepatide acts on two receptors, GIP and GLP-1, so it is a dual agonist.3 It is grouped with GLP-1 medicines in everyday use. In the EU, both its diabetes and its weight-management uses are sold as Mounjaro. Zepbound is the US weight-management name. For how it differs in practice, see our Mounjaro constipation guide.

Are there GLP-1 pills?

Yes. Rybelsus (oral semaglutide) treats type 2 diabetes. It is taken on an empty stomach with up to 120 ml of plain water, followed by at least 30 minutes without food, drink or other oral medicines.7 A Wegovy tablet for weight management was authorised in the EU in July 2026.4 See GLP-1 pills in Europe and our oral GLP-1 tracker.

How often do you take a GLP-1?

Wegovy and Mounjaro are injected once a week. Wegovy starts at 0.25 mg and rises every 4 weeks over 16 weeks to 2.4 mg.2 Mounjaro starts at 2.5 mg and rises in 2.5 mg steps, at least 4 weeks apart, up to a maximum of 15 mg, and can be taken at any time of day, with or without meals.3 Liraglutide and the tablets are taken daily. Our stage-by-stage guide walks through the dose steps.

Who can get a GLP-1 for weight loss in the EU?

Wegovy and Mounjaro are authorised “as an adjunct to a reduced-calorie diet and increased physical activity” in adults with a BMI of 30 or more, or 27 to under 30 with at least one weight-related condition. The labels give examples such as prediabetes or type 2 diabetes, high blood pressure, high blood lipids, sleep apnoea and heart disease.23 Wegovy is also authorised for adolescents aged 12 and over with obesity who weigh more than 60 kg.2

How they work and what the trials found

How do GLP-1 medicines work?

They act on appetite and on blood sugar. The Wegovy label says semaglutide “reduces energy intake, increases feelings of satiety, fullness and control of eating, reduces feelings of hunger, and frequency and intensity of cravings”.2 They also slow how fast the stomach empties.23 In one 20-week trial, people on semaglutide 2.4 mg ate 35% less energy at a free-choice meal than people on placebo.8

How much weight do people lose on GLP-1 medicines?

In the trials, alongside a reduced-calorie diet and more physical activity:

  • STEP 1 (semaglutide 2.4 mg, 68 weeks, 1,961 adults): body weight fell by 14.9% on average, against 2.4% on placebo.2
  • SURMOUNT-1 (tirzepatide, 72 weeks, 2,539 adults): body weight fell by 16.0%, 21.4% and 22.5% at 5, 10 and 15 mg, against 2.4% on placebo.3

The Wegovy label notes that weight loss was greater on average in women, in people without type 2 diabetes and in people with a lower starting weight.2 For what the months look like, see weight loss on GLP-1 medicines, stage by stage.

How fast do GLP-1 medicines start working?

The label says weight loss in STEP 1 “occurred early and continued throughout the trial”.2 No label gives a week by which appetite changes are felt. In a pooled analysis of tirzepatide trials, the median time to a weight plateau was 24 to 36 weeks, depending on starting BMI.9 Our plateau guide explains why it happens.

What happens when you stop taking a GLP-1?

Weight tends to come back. In the year after people stopped semaglutide 2.4 mg in STEP 1, weight rose in both groups, and the former semaglutide group ended 5.6% below their starting weight on average, against 0.1% for placebo.2 In SURMOUNT-4, people switched from tirzepatide to placebo regained 14.8%, while those who kept taking it lost a further 6.7%.3 See weight maintenance after GLP-1.

Do GLP-1 medicines cause muscle loss?

Some lean mass is lost along with fat. In the body-scan sub-studies on both European labels, fat mass fell more than lean mass, and most of the weight lost was fat, including visceral fat.23 Our post on fiber and muscle loss on GLP-1 covers protein and strength training. Our guide to exercise on GLP-1 covers how to train while on the medicine.

Side effects

What are the most common side effects?

Stomach and gut effects. On the European Wegovy label, nausea, vomiting, diarrhoea, constipation, abdominal pain, headache and fatigue are all very common, meaning at least 1 in 10 people.2 Over 68 weeks on semaglutide 2.4 mg, the rates were:2

  • nausea: 43.9% (placebo 16.1%)
  • diarrhoea: 29.7% (placebo 15.9%)
  • vomiting: 24.5% (placebo 6.3%)
  • constipation: 24.2% (placebo 11.1%)

Most events were mild to moderate, and 4.3% of people stopped treatment because of stomach side effects.2 On Mounjaro, nausea, diarrhoea, vomiting, abdominal pain and constipation are very common.3 For what people report online, see our study of 410,000 Reddit posts.

How long does nausea last on a GLP-1?

On semaglutide, the median episode of nausea lasted 8 days, vomiting 2 days and diarrhoea 3 days.2 These are per episode, and a new dose step can bring a new one. On tirzepatide, nausea, vomiting and diarrhoea were more frequent while the dose was being raised and decreased over time.3 See GLP-1 nausea: how long it lasts and what helps.

Why do GLP-1 medicines cause constipation or diarrhoea?

The cause is not fully worked out. A 2024 review notes that slower stomach emptying “does not fully account for” diarrhoea or constipation, which are likely driven by different mechanisms.1 The two behave differently: on semaglutide, the median constipation episode lasted 47 days, against 3 days for diarrhoea.2 See our guides to GLP-1 constipation and GLP-1 diarrhea.

Does a GLP-1 make you tired?

It can. Fatigue is listed as very common on the Wegovy label and as common in the Mounjaro weight-management trials.23 Our fatigue guide covers what the labels say.

What serious side effects should you know about?

The European labels name a few, each with a clear action:

  • Dehydration. Nausea, vomiting and diarrhoea can cause dehydration, which in rare cases can affect the kidneys. The labels ask patients to avoid fluid loss.23
  • Pancreatitis. Patients should know its symptoms, and the medicine is stopped if it is suspected. Confirmed acute pancreatitis occurred in 0.2% of people on semaglutide against under 0.1% on placebo.2 Severe stomach pain that does not go away, sometimes spreading to the back, needs prompt medical attention.
  • Gallstones. These occurred in 1.6% of people on semaglutide against 1.1% on placebo.2
  • Surgery and sedation. Food can stay in the stomach longer, so tell the medical team you take a GLP-1 before a procedure.23
  • Pregnancy. The Mounjaro label says to stop at least 1 month before a planned pregnancy.3

Contact your prescriber or pharmacist if any of these come up.

Buying, prescriptions and cost

Can you buy a GLP-1 without a prescription?

No. They are prescription-only medicines, and online sale of prescription-only medicines is not allowed in every EU country.10 Registered online pharmacies in the EU show a common logo that links to the national register, and the EMA advises not to buy if the seller is not on that list.11 In 2023, falsified Ozempic pens containing insulin were found at wholesale level in Germany; the EMA said there was no evidence any had reached patients through legal pharmacies.12 See how to spot a fake GLP-1 pen.

Are GLP-1 medicines covered by health insurance in Europe?

For type 2 diabetes, broadly yes. For weight management, mostly no. As of October 2026, public funding for weight-management GLP-1s exists in 2 of the 16 countries we track (France and Switzerland), with restricted access in the UK through NHS criteria and none in the other 13, including Germany, Spain and Italy.13 Our coverage tracker has each country, and the UK NHS access guide explains the British gates.

Food, fiber and GLP-1

Can food or supplements increase GLP-1 naturally?

A little, and briefly. Your gut releases GLP-1 after meals, and protein, some fats and the short-chain fatty acids made when gut bacteria ferment fibre can all raise it.1 In human trials the results are mixed: a 2026 scoping review found beta-glucans raised circulating GLP-1 in 3 of 9 studies, and most studies were short, single-meal tests.14 Because your own GLP-1 lasts about a minute and a half, the effect is much smaller and shorter-lived than the medicines. See our GLP-1 and gut microbiome review.

Does oatzempic work like Ozempic?

There is no evidence that it does. No scientific study has tested the “oatzempic” oat drink for weight loss, appetite or other health outcomes,15 and the EU has authorised no satiety, appetite or weight claim for oat beta-glucan.16 Our oatzempic explainer goes through the trials that do exist.

Should you eat more fiber on a GLP-1?

It is worth planning for. There is no GLP-1-specific fibre target, and EFSA’s adequate intake for adults is 25 g a day.17 Eating less makes that harder to reach, and constipation is one of the most common side effects. During active nausea, gentler and lower-fibre foods are usually suggested first,1 so increase fibre gradually and drink enough. See how much fiber on a GLP-1.

Can you take fiber with a GLP-1?

Yes, with one timing rule for pills. Weekly injections go under the skin, so fibre in the gut has no documented interaction with them. The spacing rule applies to other oral medicines: one UK psyllium label says to take it at least half an hour to an hour before or after other medicines.18 With oral semaglutide, the morning fasting window comes first and fibre goes later in the day.7 See can you take fiber with a GLP-1? and oral Wegovy fasting rules.

For the basics of fibre itself, from what it is to how much is in food, see our fiber FAQ.

Footnotes

  1. Dietary recommendations for the management of gastrointestinal symptoms in patients treated with GLP-1 receptor agonist. Diabetes, Metabolic Syndrome and Obesity (December 2024). PMC11668918. ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  2. European Medicines Agency. Wegovy (semaglutide), EPAR product information, sections 4.1, 4.2, 4.4, 4.8, 5.1, 5.2. ema.europa.eu. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13 ↩14 ↩15 ↩16 ↩17 ↩18 ↩19 ↩20 ↩21 ↩22 ↩23 ↩24 ↩25 ↩26

  3. European Medicines Agency. Mounjaro (tirzepatide), EPAR product information, sections 4.1, 4.2, 4.4, 4.5, 4.6, 4.8, 5.1. ema.europa.eu. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13 ↩14 ↩15

  4. European Commission. Union Register of medicinal products, Wegovy EU/1/21/1608, Commission Decision C(2026) 5102 of 14 July 2026. ec.europa.eu. ↩ ↩2

  5. The Good Fiber Company. Oral GLP-1 pills in Europe tracker, data current as of November 2026. ↩

  6. MHRA / emc. Ozempic (semaglutide) summary of product characteristics, section 4.8. medicines.org.uk. ↩

  7. European Medicines Agency. Rybelsus (oral semaglutide), EPAR product information, section 4.2. ↩ ↩2

  8. Friedrichsen M, et al. The effect of semaglutide 2.4 mg once weekly on energy intake, appetite, control of eating, and gastric emptying in adults with obesity. Diabetes, Obesity and Metabolism (2021). PMID 33269530. ↩

  9. Horn DB, et al. Time to weight plateau with tirzepatide. Clinical Obesity (2025). PMID 39800653, DOI 10.1111/cob.12734. ↩

  10. European Medicines Agency and Heads of Medicines Agencies. Warning about sharp rise in illegal medicines sold in the EU (3 September 2025). ↩

  11. European Medicines Agency. Buying medicines online. ema.europa.eu. ↩

  12. European Medicines Agency. EMA alerts EU patients and healthcare professionals to reports of falsified Ozempic pens (18 October 2023). ema.europa.eu. BfArM, Fälschung des Arzneimittels Ozempic (2023 to 2024). ↩

  13. The Good Fiber Company. Who pays for weight-loss GLP-1s in Europe, 16 countries, data current as of November 2026. Each row cites its national source. ↩

  14. de Jong JCB, et al. Dietary fibers to boost endogenous GLP-1 secretion and satiety: a scoping review. Frontiers in Endocrinology (2026). DOI 10.3389/fendo.2026.1880500. ↩

  15. Ball L, Burch E. What is oatzempic? Does it actually work for weight loss? The Conversation (4 January 2026). theconversation.com. ↩

  16. Commission Regulation (EU) No 432/2012, list of permitted health claims (consolidated). eur-lex.europa.eu. ↩

  17. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for carbohydrates and dietary fibre. EFSA Journal (2010);8(3):1462. ↩

  18. MHRA / emc. Ispaghula husk summary of product characteristics, section 4.5. medicines.org.uk. ↩