What is a GLP-1? A plain-English guide to how these medicines work

If you have landed here, someone has probably told you to “get on a GLP-1,” or you have read the word a hundred times without anyone stopping to say what it means. This page does that — in plain English, with no sales pitch. It is the map; the deeper pages are the territory.

GLP-1 is a hormone you already make

Start with the thing itself, before any medicine. GLP-1 stands for glucagon-like peptide-1. It is a hormone — a chemical message — that your small intestine releases into your bloodstream after you eat. Its job is to help your body handle the meal you just had. It does four main things:

• it nudges the pancreas to release insulin (which helps move blood sugar into your cells), but only when blood sugar is rising;
• it tells the body to ease off on glucagon, a hormone that raises blood sugar;
• it slows how fast your stomach empties, so food is released more gradually; and
• it acts on the brain to make you feel full and satisfied with less food.

Here is the catch that explains everything about the drugs: natural GLP-1 is destroyed almost immediately. An enzyme called DPP-4 clips it apart within a minute or two, so the message is brief by design. That is fine for your body — a signal should stop when the job is done — but it means the natural hormone itself could never work as a medicine. It would be gone before it did anything.

The medicines are engineered mimics

A “GLP-1 medicine” (more precisely, a GLP-1 receptor agonist) is a drug built to solve exactly that problem. Think of the natural hormone as a key that fits a specific lock — a receptor — on your cells. When the key turns the lock, the cell responds. These drugs are engineered keys: they fit the same lock and turn it the same way, but they are redesigned so the DPP-4 enzyme can’t clip them. Instead of lasting minutes, they last days. That is why most are taken once a week.

That single idea — mimic the hormone, but make it last — is the whole class. Everything else is variations on it.

It’s a family, not one drug

“GLP-1” is often used as if it named a single medicine. It doesn’t. It names a family, and the differences matter:

• Semaglutide — sold as Ozempic (diabetes) and Wegovy (weight management, and cardiovascular risk reduction), plus an oral tablet, Rybelsus. A pure GLP-1 receptor agonist.
• Tirzepatide — sold as Mounjaro (diabetes) and Zepbound (weight management, and obstructive sleep apnea). This one is a dual agonist: it hits the GLP-1 receptor and a second one, GIP — which is part of why its trials show larger average weight loss.
• Liraglutide — an older, once-daily GLP-1 (Victoza/Saxenda).
• Orforglipron — a newer pill (Foundayo), approved in 2026 — notable because it’s a small molecule you can swallow without the food-and-water timing rules of Rybelsus.

Beyond these are drugs still investigational — the triple-agonist retatrutide (not yet filed with the FDA) and the GLP-1-plus-amylin combination CagriSema (under FDA review, not yet approved) — which are not approved and not something you can legitimately get outside a trial. If the brand names are a blur, our brand decoder and semaglutide vs tirzepatide pages untangle which is which.

What they’re used for, and how well they work

Two approved uses anchor the class: type 2 diabetes (where these drugs improve blood-sugar control — they manage it, they don’t cure it) and chronic weight management in obesity or overweight-with-a-complication. Some products carry additional approved uses — Wegovy for reducing cardiovascular risk in people with established heart disease, Zepbound for obstructive sleep apnea. Which use a given product is approved for is specific to that product, and it drives everything from insurance coverage to what your clinician can prescribe.

On weight, the honest headline is that these are the most effective medicines yet approved for it — in the large registrational trials, average loss ran roughly 15% of body weight for semaglutide 2.4 mg (the STEP program) and around 20% for tirzepatide (SURMOUNT-1) — but “average” is doing real work: individuals land all over that range.

Retatrutide~28%

Tirzepatide (Zepbound)~20%

CagriSema~20%

Semaglutide (Wegovy)~15%

Orforglipron pill~11%

Striped bars are investigational (retatrutide, CagriSema) — not approved. Each figure is the average from that drug's own trial in different people — not a head-to-head comparison — and averages hide a wide range. Sources: STEP 1, SURMOUNT-1, REDEFINE 1, TRIUMPH-1 (topline), ATTAIN — see each drug's linked page.

Our [what-to-expect tool](/tools/what-to-expect) shows the actual responder odds, and the [non-scale-victories](/articles/non-scale-victories) piece covers the signs beyond the scale.

Side effects, briefly

The most common side effects are gastrointestinal — nausea, constipation, and sometimes vomiting — usually worst when starting or increasing the dose, and typically managed by escalating slowly. Most are mild to moderate and settle over time, but a few symptoms (severe abdominal pain, relentless vomiting, signs of gallbladder trouble) are reasons to seek care rather than wait. The full picture, including the rarer risks and the red-flag list, is on our side-effects page. Like all of this, whether one of these medicines is right for you is a conversation for you and a clinician — this page is education, not medical advice.

The one thing “GLP-1” does not tell you

Here is the literacy point worth leaving with. “GLP-1” describes how a drug works — its mechanism — and nothing else. It says nothing about whether a specific product is safe, pure, effective, or legal. An FDA-approved medicine made under federal oversight and an unregulated vial sold online as a “research peptide” can both, accurately, be called GLP-1s. That shared word is exactly what a lot of marketing leans on. The category is real science; the halo it lends to any particular vial is not. What matters is not whether something is a GLP-1, but which rung of the evidence ladder — and which link of the supply chain — a specific product actually sits on.