When you eat, your gut releases a hormone called GLP-1 (glucagon-like peptide-1). It does several 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 such as semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) are lab-made versions of that hormone. Your own GLP-1 breaks down in minutes. These drugs are engineered to last about a week, so you get a steady, around-the-clock version of the signal where you'd normally have a brief blip after each meal. The sensation is a familiar one, turned up and left on. (Strictly speaking, semaglutide is a GLP-1 receptor agonist, and tirzepatide is a dual agonist that also acts on the receptor for a second gut hormone, GIP. More on that below.)
What that steady signal 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.
- Appetite drops in your brain. GLP-1 acts on the brain areas that regulate hunger and reward, so on top of feeling 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, because the same appetite circuitry is 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 (Drucker, Cell Metabolism, 2018, DOI (external link)).
What tirzepatide adds
Semaglutide targets one receptor, GLP-1. Tirzepatide targets two: GLP-1 and the receptor for a second gut hormone called GIP, which is why it's called a dual agonist. On average it produces more weight loss. In SURMOUNT-5, the first head-to-head trial in adults with obesity but without type 2 diabetes, people on tirzepatide lost 20.2% of their body weight at 72 weeks, versus 13.7% on semaglutide (Aronne et al., NEJM, 2025, DOI (external link)). Which medication fits you is a conversation for your prescriber; the core difference between the two is one hormone versus two.
Where the side effects come from
The signal that makes you eat less is also behind the most common side effect, nausea. Slower stomach emptying is likely one contributor, since food moving through more slowly can leave you queasy. GLP-1 also acts directly on areas of the brain involved in nausea, and with long-acting drugs the slowing of the stomach tends to lessen over time, so it isn't the whole story (Drucker, 2018). Doctors start you on a low dose and raise it gradually, a process called titration, to give your body time to adjust instead of hitting it with the full effect on day one. For what to do about nausea and other gut symptoms, see the GI side effects overview.
Knowing the mechanism takes some of the mystery out of what comes next. Less hunger, feeling full faster, quieter food thoughts, and occasional queasiness all trace back to one signal. For how that plays out week by week, see what to realistically expect on a GLP-1.
What it doesn't do
The medication works on calories in: how much you eat. It does nothing directly to protect muscle or keep your metabolism up as you lose weight, which is why nutrition and strength work still matter. It helps most with the hardest part of weight loss, the constant hunger, and leaves the rest to you.
Study findings above are attributed to their authors and were located via PubMed.
This is general education, not medical advice. Talk to your prescriber about which medication and dose fit your situation.