How GLP-1 drugs actually work

GLP-1 stands for glucagon-like peptide-1. It is a hormone, itself a short peptide, made in the gut and released into the blood within minutes of eating. Its job is to tell the rest of the body that food has arrived and to coordinate the response: get ready to handle the incoming glucose, and signal the brain that a meal is underway.

Natural GLP-1 is broken down within a couple of minutes, so the body's own version is short-lived. GLP-1 drugs are engineered to resist that breakdown, so a single dose keeps working for days. That is the core trick: take a useful natural signal and make it last. Medicines in this class include semaglutide, liraglutide, dulaglutide and exenatide.

Once a GLP-1 drug is in the system, it does four distinct things at once:

• Slows the stomach. Food leaves the stomach more slowly, so you feel full for longer after eating and blood sugar rises more gently.
• Sharpens insulin, but only when needed. It nudges the pancreas to release insulin in response to a rise in blood sugar. This is glucose-dependent, which is why GLP-1 drugs on their own rarely cause dangerously low blood sugar, they do not force insulin out when sugar is already normal.
• Quietens glucagon. Glucagon is the hormone that tells the liver to pump glucose into the blood. GLP-1 turns it down when blood sugar is high, so the liver stops adding fuel that is not needed.
• Reduces appetite in the brain. Receptors in the brain's appetite centres respond to the signal, so hunger drops and you feel satisfied with less food.

For type 2 diabetes, the insulin and glucagon effects matter most. For weight management, the slowed stomach and reduced appetite do most of the work.

GLP-1 is an incretin, one of the gut hormones that explains why glucose taken by mouth triggers far more insulin than the same glucose given by drip. It is released by L-cells in the intestine and acts on the GLP-1 receptor, a G-protein-coupled receptor found on pancreatic beta cells, in the stomach and gut, and in regions of the brain including the hypothalamus and hindbrain.

Natural GLP-1 is degraded almost immediately by an enzyme called DPP-4, giving it a half-life of roughly one to two minutes. GLP-1 receptor agonists are modified to evade DPP-4 and, in the longer-acting versions, to bind to albumin in the blood, which stretches their half-life from minutes to about a week for the weekly injections. The downstream effect on beta cells is glucose-dependent insulin release: the receptor amplifies insulin secretion only when glucose is already elevated, which is the molecular reason these drugs carry a low intrinsic risk of hypoglycaemia.

GLP-1 is not the only incretin. GIP (glucose-dependent insulinotropic polypeptide) is a second gut hormone with overlapping effects, and glucagon receptors, when engaged deliberately, can raise energy expenditure. The newer drugs hit more than one of these targets at once.

Tirzepatide is a dual agonist: it activates both the GLP-1 and the GIP receptor. Retatrutide is a triple agonist in development, adding the glucagon receptor on top. The logic is that combining receptors can produce larger effects on blood sugar and weight than targeting GLP-1 alone. A related approach pairs a GLP-1 drug with cagrilintide, an amylin analogue, to layer a second satiety signal on top. These are genuinely different molecules with different evidence, and combining receptors does not automatically mean safer, only that the levers being pulled are different.

Because these drugs slow the stomach and act on the gut, the most common side effects are nausea, vomiting, diarrhoea and constipation, especially when starting or increasing a dose. For most people these ease over time, which is why prescribers raise the dose gradually rather than starting high.

It is just as important to be clear about what GLP-1 drugs do not do. They are not a stimulant and they do not "burn fat" directly; the weight effect comes mainly from eating less because you are less hungry and feel full sooner. They are not a cure, blood sugar and weight tend to return when the drug is stopped, because the natural signal it was amplifying goes back to baseline. They do not selectively remove fat, and they are not a substitute for the broader picture of diet, activity and clinical care. This page explains mechanism only and is not medical advice; for the legal picture see are GLP-1 medicines legal in the UK, and see how we grade evidence for our approach.