How GLP‑1 medications work, in plain words

What is GLP-1?

GLP-1 is a hormone your gut releases after you eat, one of the signals that tell your body food has arrived. According to the US labels, it helps regulate appetite and calorie intake, and its receptor is present in brain areas involved in appetite. It also tells the pancreas to release insulin, but only when blood sugar is raised.

Your own GLP-1 does not last. The Saxenda label notes that it has a half-life of 1.5 to 2 minutes, because enzymes in the blood, mainly one called DPP-4, break it down almost at once. That is far too short to use as a medicine.

What is a GLP-1 receptor agonist?

A GLP-1 receptor agonist is a man-made molecule that switches on the GLP-1 receptor like your own hormone but resists being broken down. "Agonist" simply means something that activates a receptor.

Liraglutide shares 97% of its sequence with human GLP-1 and semaglutide 94%. The Ozempic label says semaglutide lasts so long mainly because it binds to albumin, a blood protein, which slows clearance by the kidneys and protects it from breakdown. The result is a steady version of the "I've eaten" signal, rather than the brief burst that follows a meal.

How do GLP-1 medicines reduce appetite and weight?

They mainly work in the brain, by lowering appetite and how much you eat. The Wegovy and Zepbound labels say both medicines decrease calorie intake, that the effect is likely through appetite, and that in animal studies they reached and activated neurons in brain regions that regulate food intake.

Day to day, people describe feeling full sooner and thinking about food less. Both labels say weight loss included more fat than lean mass, but some muscle is still lost, which is why protein and strength training matter.

These are averages, and results vary widely. The labels pair these medicines with a reduced-calorie diet and more physical activity. The weight timeline tool shows the trial curves over time.

How do they affect the stomach and digestion?

They slow gastric emptying, so food stays in your stomach longer. That adds to fullness and helps explain the most common side effects: nausea, burping, reflux, constipation and diarrhea. The Zepbound label says the delay is largest after the first dose and shrinks over time. Because of it, tell your care team you take one of these medicines before any procedure that needs anesthesia or deep sedation.

How do they affect blood sugar and insulin?

They help the pancreas release more insulin and less glucagon when blood sugar is high, and the effect fades as glucose comes down. This is the effect behind Ozempic and Mounjaro, the type 2 diabetes brands.

On their own, these medicines rarely cause low blood sugar. In the Zepbound trial in people without diabetes, a glucose below 54 mg/dL was recorded in 0.3% vs none on placebo. The risk is higher in people with type 2 diabetes who also take insulin or a sulfonylurea, and the labels tell prescribers to consider lowering those doses.

What does tirzepatide's GIP add?

Tirzepatide also activates the receptor for GIP (glucose-dependent insulinotropic polypeptide), another gut hormone released after meals. It is one molecule with two actions, not two drugs mixed together. The Zepbound label says nonclinical studies suggest GIP may add to the regulation of food intake, that both receptors are found in brain areas involved in appetite, and that tirzepatide increases insulin sensitivity.

In SURMOUNT-5, the head-to-head trial in adults with obesity, tirzepatide led to more average weight loss than semaglutide (-20.2% vs -13.7% at 72 weeks). The semaglutide vs tirzepatide guide covers that trial in detail.

Why are most GLP-1 shots once a week?

Because semaglutide and tirzepatide stay in the body for days, so each weekly shot tops up a level still there from the week before.

The Wegovy label says semaglutide stays in the blood for about 5 to 7 weeks after the last dose.

The semaglutide pills (the Wegovy pill, Ozempic tablets and Rybelsus) are daily because little is absorbed through the stomach: about 1% to 2%, according to the Wegovy label. That is why the pill dose (25 mg) is much higher than the injection dose (2.4 mg). The tablets contain an absorption helper called SNAC and are taken on an empty stomach with a little water, at least 30 minutes before food.

Why is the dose stepped up slowly?

The labels say the gradual increase is there to reduce the risk of stomach side effects. Wegovy injection starts at 0.25 mg weekly and rises every 4 weeks (0.5, 1, 1.7 mg) to a maintenance dose of 1.7 or 2.4 mg from week 17, and some adults may later move to 7.2 mg after at least 4 weeks on 2.4 mg. Zepbound starts at 2.5 mg weekly for 4 weeks, then 5 mg, with 2.5 mg increases allowed after at least 4 weeks on a dose, up to 15 mg. The Wegovy pill goes 1.5, 4, 9 and then 25 mg daily, each step lasting 30 days.

This is what the labels describe, not a plan for you. Your prescriber sets your doses and may hold a step longer if side effects are hard. The titration schedules tool shows each label schedule side by side.

What don't GLP-1 medicines do?

They do not change how your body works permanently. When the medicine stops, the effect fades. In the STEP 1 extension, people who stopped semaglutide 2.4 mg after 68 weeks regained about two-thirds of their lost weight within a year. In SURMOUNT-4, people switched from tirzepatide to placebo regained 14% of their weight over the next year, while those who stayed on it kept losing.

They are also not:

• A substitute for food quality. Eating less makes protein, fiber and fluids more important, not less.
• A metabolism booster. The labels say the weight loss likely comes from lower appetite and calorie intake.
• Something to combine. The labels say not to use two GLP-1 medicines together.
• For everyone. Both semaglutide and tirzepatide carry a boxed warning about thyroid C-cell tumors seen in rodents and should not be used by people with a personal or family history of medullary thyroid cancer or MEN 2.

Get medical care right away for severe stomach pain that will not go away (with or without vomiting), signs of an allergic reaction such as swelling of the face, lips or throat, or signs of dehydration such as very little urine or dizziness. Tell your clinician about a lump in your neck, trouble swallowing or ongoing hoarseness.

Which brands are GLP-1 medications?

Three molecules cover the main brands: semaglutide and liraglutide from Novo Nordisk, and tirzepatide from Eli Lilly. The brand tells you the approved use; the molecule is what matters medically. The same molecule guide explains why Wegovy and Ozempic, or Zepbound and Mounjaro, are the same drug.

The Wegovy pill was approved by the FDA in December 2025, and Ozempic tablets in February 2026. Ozempic tablets and Rybelsus are both oral semaglutide for type 2 diabetes, but the label says their strengths are not interchangeable mg for mg. Zepbound is a US brand; in Mexico and much of Latin America, tirzepatide is sold as Mounjaro.

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Educational information, not medical advice. Talk to your clinician about your treatment.