The science, in plain terms
How do weight-loss injections work?
Wegovy, Saxenda and Mounjaro are not appetite suppressants in the old sense. They copy a hormone your gut already makes, and they work by changing signals rather than by burning anything off.
Start with the hormone, not the drug
GLP-1 — glucagon-like peptide-1 — is a hormone your gut releases after you eat. It helps regulate appetite and blood sugar. Left alone, it breaks down within minutes.
These medicines are GLP-1 receptor agonists: molecules shaped closely enough to bind the same receptor, but built to last hours or days rather than minutes. That difference in duration is the whole idea. Everything below follows from it.
What the medicine does, in three moves
They happen at the same time, not in sequence. Splitting them apart just makes each one easier to recognise in yourself.
01
It tells your brain you are full
The drug reaches the hypothalamus, the part of the brain that governs hunger, and signals fullness. That is why people describe appetite and food noise as quietly receding rather than as something they are holding back by willpower.
02
It slows the stomach down
Gastric emptying — the rate at which food leaves your stomach for the small intestine — slows down. Meals sit longer, so fullness lasts longer. It is also where most of the early nausea comes from, which is why doses start low and climb slowly.
03
It steadies blood sugar
It prompts the pancreas to release insulin in proportion to how high blood sugar actually is, rather than regardless of it. This is the original purpose of the class — these drugs were developed for type 2 diabetes, and weight loss arrived as a consequence.
Mounjaro adds a second hormone
This is the one real mechanical difference between the three medicines sold in the UK.
One hormone
GLP-1 only
Wegovy (semaglutide) and Saxenda (liraglutide) act on the GLP-1 receptor alone. The three moves above are the whole mechanism.
Two hormones
GLP-1 and GIP
Mounjaro (tirzepatide) also acts on the GIP receptor — glucose-dependent insulinotropic polypeptide, a second gut hormone involved in the same signalling.
In trials, the dual-receptor approach has been associated with greater average weight loss. Average is doing real work in that sentence: two mechanisms is not automatically the better choice for any given person, and tolerability, dosing schedule and cost all point in their own directions. We break the comparison down in GLP-1 vs GIP.
Knowing the mechanism is not the same as knowing it suits you
This page explains what the drug does in a body. It cannot tell you whether it should be in yours — that depends on your history, what else you take, and conditions that rule these medicines out entirely. Those are questions for a prescriber, and the assessment exists for a reason.
What this page is built on
- Mechanisms of action of GLP-1 receptor agonists — Diabetes Care
- Semaglutide for weight management — New England Journal of Medicine
- The Summary of Product Characteristics and patient information leaflet for each product, held by the MHRA
This page does not carry a reviewer’s name, so it has not had clinical sign-off. Our editorial policy explains what that label means and when we use it.