What Is GLP‑1? The Hormone, the Medications and How They Work

GLP-1 stands for glucagon-like peptide-1. It is a peptide hormone released from enteroendocrine cells in the gut.1 The "glucagon-like" part of the name reflects its family, not its job: GLP-1 actually lowers glucagon secretion when blood sugar is high.1,3

After a meal, natural GLP-1 boosts insulin release, cuts glucagon, slows gastric emptying and reduces food intake.1 Its effect on insulin is glucose-dependent: GLP-1 receptor activation releases insulin when blood glucose is elevated, and that secretion subsides as glucose returns toward normal.3 Researchers have also linked GLP-1 to reward behavior and food palatability.2

Natural GLP-1 does not last long. The active form has a half-life of 1.5 to 2 minutes, because the enzymes DPP-4 and neutral endopeptidase break it down.3 That short life is why the hormone itself cannot work as a medicine.

A GLP-1 medication, or GLP-1 receptor agonist, is a drug that binds to and activates the GLP-1 receptor but resists breakdown, so it keeps working for hours or days.3,4 Liraglutide (Victoza, Saxenda) has a half-life of about 13 hours and is injected daily.3 Semaglutide (Ozempic, Wegovy) shares 94% of its sequence with human GLP-1, is modified to resist DPP-4, and has a half-life of about 1 week.4

Some newer drugs act on more than one receptor. Tirzepatide (Mounjaro, Zepbound) activates both the GIP and GLP-1 receptors.10 Orforglipron (Foundayo) is a GLP-1 receptor agonist taken as a daily tablet.11

GLP-1 drugs mainly cause weight loss by reducing appetite and food intake, acting on GLP-1 receptors in brain areas that regulate appetite.4 The Wegovy label describes GLP-1 as "a physiological regulator of appetite and caloric intake," and animal studies show semaglutide reaching and activating neurons in those brain regions.4 Foundayo's label states that the drug decreases food intake, likely through decreased appetite, and delays gastric emptying.11

The size of the effect, from each drug's label:

• Wegovy 2.4 mg injection: 14.9% average weight loss at 68 weeks, against 2.4% on placebo.4
• Wegovy 25 mg tablet: 13.6% at 64 weeks, against 2.4% on placebo.4
• Zepbound 15 mg: 20.9% at 72 weeks, against 3.1% on placebo.10
• Foundayo 17.2 mg: 11.1% at 72 weeks in adults without diabetes, against 2.1% on placebo.11

Trials differ in design, so these figures are not a head-to-head ranking. For a direct comparison of the two leading weekly injections, see Wegovy vs Zepbound side by side and our explainer on what food noise is.

The main FDA-approved GLP-1 medications in the US are made by Novo Nordisk and Eli Lilly and contain five molecules: semaglutide, liraglutide, dulaglutide, tirzepatide and orforglipron. The table lists the main brands with their approved uses, taken from each current label.

Weight-loss approvals sit with Wegovy, Saxenda, Zepbound and Foundayo; Ozempic, Rybelsus, Victoza, Trulicity and Mounjaro are approved for type 2 diabetes.4,7,10,11,5 Compounded semaglutide and tirzepatide are not on this list: the FDA states that compounded drugs are not FDA approved.12 To see how the weight-loss options compare on price and access, see our best GLP-1 programs and GLP-1 pills vs injections.

Yes. Ozempic and Wegovy are GLP-1 receptor agonists, and Mounjaro and Zepbound are dual GIP and GLP-1 receptor agonists.5,4,9,10 Tirzepatide, the molecule in Mounjaro and Zepbound, is therefore a GLP-1 drug with a second target.10

The most common side effects are gastrointestinal: nausea, diarrhea, vomiting, constipation and abdominal pain.4,10 The Wegovy label also lists headache, fatigue, dyspepsia, dizziness, bloating, burping, reflux, flatulence and hair loss among reactions seen in at least 5% of patients.4 Most labels raise doses gradually to reduce stomach side effects.4,10

Serious risks named in the labels include:

• Thyroid C-cell tumors: a boxed warning based on rodent studies; the drugs are contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2.4
• Acute pancreatitis and gallbladder disease.4
• Low blood sugar when combined with insulin or a sulfonylurea.4
• Kidney injury from dehydration after vomiting or diarrhea.4
• Aspiration during anesthesia, so patients should tell their surgical team they take one.4

In January 2026 the suicidal behavior and ideation warning was removed from the Wegovy label.4 Our drug-specific guides cover the full tables: Ozempic side effects, Zepbound side effects and Wegovy side effects.

No. Metformin is a biguanide that lowers liver glucose production, reduces glucose absorption from the gut and improves insulin sensitivity; it does not act on the GLP-1 receptor.15 Its label notes that its mechanisms differ from other classes of oral diabetes drugs.15 See metformin vs GLP-1 drugs for the full comparison.

No. Phentermine is a sympathomimetic amine, a stimulant related to amphetamine, approved only for short-term weight loss and classed as a Schedule IV controlled substance.14 It works through the nervous system, not the GLP-1 receptor.14 See phentermine vs GLP-1 drugs.

Retatrutide acts on the GLP-1 receptor, but it is an investigational drug that also targets the GIP and glucagon receptors, and it is not FDA-approved.13 After Phase 3 results in July 2026, Lilly said it planned to file with the FDA in the first quarter of 2027.13 Our retatrutide explainer covers the trial data.

Do you need a prescription for a GLP-1? Yes. Every product in the table above is a prescription drug with FDA-approved prescribing information.4,10,11 Products sold without a prescription are not FDA-approved GLP-1 medications.12

Are GLP-1 drugs safe? They are FDA-approved for specific uses, with known risks listed in each label, including a boxed thyroid tumor warning.4 Whether one is right for you depends on your history, which a prescriber should review.

Is there a GLP-1 pill? Yes. Wegovy tablets, Foundayo and Rybelsus (and Ozempic tablets) are taken by mouth once daily.4,11,6 Wegovy tablets are taken on an empty stomach with up to 4 ounces of water, while Foundayo can be taken with or without food.4,11 Our ranking of oral GLP-1 programs and Foundayo vs Wegovy compare the pills.

Which GLP-1 is best for weight loss? In separate trials, tirzepatide 15 mg produced the largest average loss of the approved options, at 20.9%.10 The right choice also depends on cost, form and side effects; see our brand-name GLP-1 rankings and tirzepatide vs semaglutide.

Can you take two GLP-1 drugs together? No. The Wegovy and Foundayo labels say use with another GLP-1 receptor agonist is not recommended.4,11

If you are weighing which GLP-1 to ask about, start with who qualifies for a GLP-1, then compare how Foundayo stacks up against Zepbound and the Wegovy cost and Zepbound cost guides.

This article is for information only and is not medical advice. GLP-1 medications are prescription drugs with real risks and side effects. Talk to a licensed clinician before starting or stopping any treatment.