How GLP-1 drugs work for fat loss

GLP-1 drugs copy a hormone your gut makes when you eat. It tells your brain you’ve had enough and slows down your stomach, so you feel full on less food. In the main trials, people lost about 8% of their weight on liraglutide (Saxenda), 15% on semaglutide (Wegovy) and 21% on the top dose of tirzepatide (Zepbound). They work while you take them, and they need a prescription.

How they work

1. You eat, and your gut releases GLP-1. It’s one of the signals your body uses to say a meal has arrived.
2. GLP-1 acts on the parts of your brain that control appetite. Hunger drops and you feel full sooner.
3. It slows how fast your stomach empties, so a meal stays with you longer.
4. It raises insulin only when blood sugar is high, which is how the same drugs treat type 2 diabetes.
5. The drugs are built to last. Semaglutide stays in the blood about a week, so one shot a week is enough. Tirzepatide also acts on a second gut hormone, GIP.

The drugs work like a thermostat for appetite. They turn the hunger dial down, so eating less stops feeling like a fight.

The drugs, by name

Head to head, tirzepatide did more. In SURMOUNT-5 (2025), 751 adults on the highest dose they could take lost 20.2% of their weight on tirzepatide and 13.7% on semaglutide over 72 weeks.

The weight loss brands are for adults with obesity, or with overweight and a weight-related health problem. For Zepbound, the FDA puts that at a BMI of 30 or more, or 27 or more with a condition like high blood pressure, type 2 diabetes or high cholesterol.

The drug pages have the trials and the details: Ozempic and Wegovy, Mounjaro and Zepbound, Saxenda.

What happens to your muscle

Some of the weight lost is lean mass, as with any big weight loss. In the tirzepatide trial’s scan study of 160 people, about 26% of the loss was lean mass, against 25% in the placebo group who lost weight on diet and exercise advice. In the semaglutide trial’s scan study of 140 people, the lean share was closer to 40%. Lean mass counts water and organs as well as muscle.

The 2025 advisory from four US nutrition and obesity societies suggests strength training at least three times a week, at least 150 minutes of moderate cardio, and about 1.5 g of protein per kg of lean mass a day. Find your range. More is in do GLP-1 drugs make you lose muscle.

What happens when you stop

A year after people stopped semaglutide in its main trial, two thirds of the weight they lost was back. When people on tirzepatide were switched to a placebo after 36 weeks, they regained 14% of their weight over the next year, while those who stayed on it lost another 5.5%. See what happens when you stop taking Ozempic.

The side effects that matter

Stomach side effects are common. On Wegovy 2.4 mg, 44% of people had nausea, against 16% on placebo, and 30% had diarrhea, 24% vomiting and 24% constipation. On Zepbound, nausea affected 25 to 29% depending on dose, against 8% on placebo.

The serious warnings on the labels include pancreatitis, gallbladder problems, low blood sugar when taken with insulin or some diabetes pills, and kidney injury from losing fluid through vomiting or diarrhea. All of these drugs carry a boxed warning because they caused thyroid tumors in rodents. People with a personal or family history of medullary thyroid cancer or a condition called MEN 2 shouldn’t take them. Tell your surgeon you take one before any procedure that needs anesthesia.

Beyond weight. In SELECT, 17,604 adults with heart disease and overweight but no diabetes had 20% fewer heart attacks, strokes and heart deaths on semaglutide than on placebo.