When you eat, your gut releases a hormone called GLP-1 (glucagon-like peptide-1). It does a handful of useful things at once: it tells your pancreas to release insulin, it slows how fast your stomach empties, and it signals your brain that you're full. It's part of how a normal meal ends with you pushing the plate away.
GLP-1 medications — semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) — are lab-made copies of that hormone. Your own GLP-1 breaks down in minutes. These drugs are engineered to survive for about a week, so instead of a brief post-meal blip, you get a steady, around-the-clock version of the same signal. That's the whole idea: not a new sensation, but a familiar one turned up and left on.
What that steady signal actually does
Three effects matter most for weight:
- Your stomach empties more slowly. Food sits longer, so you feel full sooner during a meal and stay full for hours after. This is the effect you notice first — and, as you'll see, it's also the source of the most common side effect.
- Appetite drops at the source — in your brain. GLP-1 acts on the parts of the brain that regulate hunger and reward. You're not just fuller; you're less interested in eating in the first place.
- "Food noise" quiets down. Many people describe a constant background chatter about food — what to eat next, the snack in the cupboard, the second helping. On these medications that chatter often fades. That's not willpower arriving. It's the same appetite circuitry being turned down.
The blood-sugar effect (prompting insulin, which is why these started as diabetes drugs) runs in the background too, but for weight loss the appetite and fullness effects are the ones you feel.