How GLP-1 medicines actually work

Every time you eat, cells in your small intestine release a hormone called glucagon-like peptide-1. It is a messenger with a short life — the body breaks it down within a couple of minutes — and its job is to tell several organs at once that food has arrived.

That short life is the whole reason this class of medicine exists. Natural GLP-1 works, but it disappears too quickly to be a treatment. The engineering problem of the last twenty years was not what to give people. It was how to make a copy that survives.

The three things it does

GLP-1 receptors sit in more than one place, and the hormone does a different job at each. This is why a single molecule produces effects that seem unrelated.

1. It makes insulin release depend on glucose

In the pancreas, GLP-1 amplifies insulin release — but only when blood glucose is already elevated. When glucose is normal, it does very little. This glucose-dependence is why GLP-1 medicines rarely cause hypoglycaemia on their own, and why they behave so differently from insulin itself. It also suppresses glucagon, the hormone that tells the liver to release stored sugar.

2. It slows the stomach down

GLP-1 delays gastric emptying. Food sits in the stomach longer, so glucose enters the bloodstream as a gentler curve instead of a spike. This is a metabolic benefit and, at the same time, the single largest source of side effects. A stomach that empties slowly is a stomach that feels full early, and sometimes a stomach that feels queasy.

3. It changes appetite in the brain

GLP-1 receptors in the hypothalamus and brainstem influence the sense of having had enough. People describe this less as willpower and more as an absence — the pull toward a second helping is simply not there. Many also report that the constant background hum of thinking about food gets quieter, an effect informally called food noise.

How the copies survive

Natural GLP-1 is destroyed in minutes by an enzyme called DPP-4. Every medicine in this class is a modification designed to resist that enzyme and to cling to albumin, a carrier protein in the blood, so the kidneys clear it slowly.

The result is dramatic. Semaglutide has a half-life of roughly 160 hours — about a week. Tirzepatide is close to five days. That is why these are weekly injections rather than daily ones, and why a single missed dose is much less consequential than it would be with most medicines.

The second hormone: what makes tirzepatide different

Semaglutide, liraglutide and dulaglutide act on the GLP-1 receptor alone. Tirzepatide — sold as Mounjaro® and Zepbound® — also activates the receptor for a second gut hormone, glucose-dependent insulinotropic polypeptide, or GIP.

GIP is released by the gut alongside GLP-1 and appears to add its own effects on insulin release, fat tissue, and appetite regulation. In head-to-head trials the dual-agonist has produced larger average weight reductions than semaglutide. That is an average across thousands of people, not a promise about any individual — response varies widely, and the medicine that works best for you is not always the one with the better trial number.

Two names for the same molecule is a licensing detail, not a medical one. Ozempic® and Wegovy® are both semaglutide, approved for different indications at different maximum doses. Mounjaro® and Zepbound® are both tirzepatide. The drug in the pen is identical; the label and the dose ceiling are not.

What they do not do

It is worth being precise about the limits, because the gap between what these medicines do and what people expect them to do causes a lot of disappointment.

• They do not burn fat. There is no metabolic furnace being switched on. Weight changes because intake falls.
• They do not choose what you lose. Weight lost quickly, on low protein and without resistance training, includes a meaningful share of lean tissue. This is the most under-discussed problem in the whole category.
• They do not retrain appetite permanently. Appetite tends to return when the medicine is stopped, because the signal you were responding to is gone.

What actually determines how this goes

Trial results are averages. What separates a good outcome from a disappointing one, in practice, tends to come down to a small number of unglamorous things: getting the dose in on schedule, moving up the dose slowly enough that side effects stay tolerable, eating enough protein to protect muscle while intake is low, and staying on long enough for the appetite effect to reach its full size.

None of that is exciting. All of it is more predictive than which brand is in your fridge.

Related

• How long GLP-1 medicines stay in your system — Half-lives, steady state, and the arithmetic behind a missed dose — including why stopping does not mean the effect ends that week.
• What to expect, month by month — A realistic timeline of the first year — what usually happens when, and which of it means something.

Which medicine you are on

Everything above applies across the class, because it comes from a shared mechanism. The numbers that differ — dose ladders, missed-dose windows, in-use storage periods, half-lives — differ by product, sometimes substantially. Each medicine has its own reference here:

Ozempic · Wegovy · Mounjaro · Zepbound · Rybelsus · Saxenda · Victoza · Trulicity · all medicines