GLP1 Europe › Basics
What is GLP-1, and what does it do in the body?
Short answer
GLP-1 is a hormone your own gut releases every time you eat. It has four main jobs: it tells the pancreas to release insulin, tells it to hold back glucagon, slows how fast the stomach empties, and signals to the brain that you have had enough.
The medicines are laboratory-made copies that survive in the body for days rather than minutes. That is the entire trick. Nothing about them is new to your body. They just keep a normal signal switched on far longer than a meal ever would.
Where GLP-1 comes from
GLP-1 stands for glucagon-like peptide-1. It is made by specialised cells, called L-cells, in the lining of the small intestine and colon. When food arrives, those cells release GLP-1 into the bloodstream within minutes.
The name is unhelpful. GLP-1 is called "glucagon-like" because of its chemical resemblance to the hormone glucagon, not because it behaves like it. In terms of what it actually does to blood sugar, it is closer to the opposite.
This is worth sitting with for a moment, because it reframes the whole subject. GLP-1 medicines are not stimulants, appetite suppressants of the old amphetamine kind, or fat blockers. They work on a signalling system that already runs in everyone, several times a day, and has done for as long as humans have eaten meals.
What GLP-1 actually does
Four effects matter. Different ones explain different parts of what the medicines do.
| Where it acts | What happens | Why it matters |
|---|---|---|
| Pancreas, beta cells | Prompts insulin release, but only when blood sugar is already raised | Lowers blood sugar after meals. The "only when raised" part is why GLP-1 drugs rarely cause hypoglycaemia on their own |
| Pancreas, alpha cells | Suppresses glucagon, the hormone that tells the liver to release stored sugar | Stops the liver adding sugar to the bloodstream at the moment food is already supplying it |
| Stomach | Slows the rate at which the stomach empties into the intestine | Food stays put for longer, so you feel full for longer and blood sugar rises more gently. Also the main source of nausea |
| Brain, hypothalamus and brainstem | Acts on the circuits that govern appetite and satiety | Reduces hunger and the pull toward food. This is thought to do most of the work in weight loss |
Two of these, the insulin and glucagon effects, are why the first GLP-1 drugs were developed as diabetes treatments. The weight loss started out as a side effect that patients and doctors noticed. Only later did it become the point.
Why the natural hormone cannot be a medicine
Natural GLP-1 lasts roughly a minute or two in the bloodstream before an enzyme called DPP-4 chops it up and it stops working.
That is not a design flaw. It is the design. A meal signal that lingered for days would be useless, because the whole point is to mark the difference between having eaten and not having eaten. The body builds in an off switch.
It does mean you cannot bottle the natural hormone and inject it. It would be gone before it did anything. Every drug in this class exists to solve that one problem.
How the medicines get around it
Drug designers made two kinds of change to the natural molecule.
They made it harder for DPP-4 to cut. Swapping a single building block at the site the enzyme attacks means the enzyme no longer recognises it properly. The molecule survives.
They made it stick to something big. Attaching a fatty acid chain lets the molecule bind loosely to albumin, an abundant protein in blood. Bound to albumin, it is too large for the kidneys to filter out quickly, and it detaches slowly, releasing a trickle of active drug over days.
Put together, these take a hormone that lasts two minutes and turn it into a drug that lasts about a week. That is why semaglutide is a once-weekly injection and older liraglutide, which uses a shorter fatty acid chain, is a daily one.
Orforglipron takes a different route entirely. It is not a modified peptide at all but a small molecule, built from scratch to fit the same receptor. Because it is not a peptide, the digestive system does not destroy it, so it can be swallowed as an ordinary tablet.
A word you will keep seeing
Receptor agonist. A receptor is a lock on the surface of a cell. An agonist is a key that fits the lock and turns it on. So "GLP-1 receptor agonist" just means a substance that fits the GLP-1 lock and switches it on, whether or not it is GLP-1 itself. The opposite, a key that fits the lock and jams it, is an antagonist.
Why the side effects are mostly digestive
Nausea, vomiting, diarrhoea and constipation dominate the side effect lists for every drug in this class. This is not a coincidence or an impurity problem. It is the mechanism.
Slowed stomach emptying is one of the four core actions. Exaggerate it and you get fullness, which is wanted, and queasiness, which is not. They are the same effect at different intensities. The brainstem areas involved in appetite also sit next to those involved in nausea, which adds a second route to the same symptom.
This is also why side effects usually cluster around dose increases and settle afterwards, and why they get worse at higher doses. In the Phase 3 retatrutide trial, the share of participants who stopped because of side effects rose steadily with the dose. Regulators weigh that trade-off, not just the headline weight loss.
What happens when you stop
Weight comes back. This is one of the most consistent findings in the field and one of the least discussed in marketing.
The STEP 4 trial was built to test exactly this. Participants took semaglutide for 20 weeks, then either continued or were switched to placebo. The group switched to placebo regained a large share of what they had lost. Reviewing the evidence in 2025, the Danish Medicines Agency put the figure at more than 60 percent of lost weight regained within 48 weeks of stopping.
This follows logically from everything above. The drug does not retrain the appetite system, it overrides it while present. Remove it and the original signalling returns. It is closer to blood pressure medication than to a course of antibiotics: it manages a condition for as long as you take it, rather than curing something and then being finished.
Common questions
- Is GLP-1 the same as insulin?
No. Insulin lowers blood sugar directly. GLP-1 is a signal that tells the pancreas to release its own insulin, and only when blood sugar is already high. That conditional quality is why GLP-1 drugs alone rarely push blood sugar too low.
- Can you raise your natural GLP-1 with food?
Eating does raise it, and protein and fibre raise it more than refined carbohydrate. But the rise is small and short compared with what a medicine produces, and no diet reproduces a week-long elevation. Claims that a particular food or supplement is "nature's Ozempic" are not supported by the size of the effect.
- What is GIP, and how is it different?
GIP is a second gut hormone released after eating, with overlapping but not identical effects. Tirzepatide activates both GLP-1 and GIP receptors. Retatrutide adds a third, glucagon. The family comparison page covers what each combination does.
Sources
- Wilding JPH, Batterham RL, Calanna S, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity" (STEP 1). New England Journal of Medicine, 2021. DOI 10.1056/NEJMoa2032183.
- Rubino D, Abrahamsson N, Davies M, et al. STEP 4 withdrawal trial. JAMA, 2021.
- Lægemiddelstyrelsen (Danish Medicines Agency), decision on general clausulated reimbursement for Wegovy, 10 July 2025, citing weight regain after discontinuation.
- European Medicines Agency, product information for Ozempic, Wegovy, Mounjaro and Saxenda, checked 14 August 2026.
- Eli Lilly and Company, TRIUMPH-1 results, 21 May 2026, for the dose-related discontinuation figures.
This page explains how a class of medicines works. It is not medical advice and it is not a recommendation to use or obtain any medicine. If you are considering treatment, speak to a doctor or a pharmacist.