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

> GLP-1 is a gut hormone your body already makes. The medicines — Ozempic, Wegovy, Mounjaro, Zepbound and the rest — are engineered mimics of it. Here's what that means, in plain English.

**Evidence grade:** Established (rung 1 of 8 on the Peptide Evidence Ladder) · **Last reviewed:** 2026-07-01 · **Source page:** https://thepeptideera.com/evidence/what-is-a-glp1

## Short answer
GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat; it nudges insulin up, glucagon down, stomach-emptying slower, and appetite lower. The medicines people call 'GLP-1s' — semaglutide (Ozempic/Wegovy), tirzepatide (Mounjaro/Zepbound), and others — are engineered drugs that switch on the same receptor as that natural hormone, but are built to last days instead of minutes. They're approved to treat type 2 diabetes and, for several products, obesity; the most common side effects are gastrointestinal.

## Key takeaways
- GLP-1 is a natural gut hormone, not a lab invention — it signals fullness and helps control blood sugar after a meal, then breaks down within minutes.
- The medicines are “receptor agonists”: engineered mimics that flip the same switch as the natural hormone but resist breakdown, so one dose lasts about a week.
- It's a family, not one drug. Semaglutide, tirzepatide, liraglutide, and the new oral orforglipron differ in what they target, how they're taken, and what they're approved for.
- “GLP-1” describes how a drug works — not whether a given product is safe or legitimate. An approved medicine and an unregulated online “peptide” can share the label; only one has oversight behind it.

## Safety essentials worth knowing

These apply to GLP-1 and dual/triple-agonist medicines generally. They’re label-level points, not the full list, and not advice about you — your prescriber and the FDA label are the authority.

- **Pregnancy & breastfeeding:** these medicines are not recommended in pregnancy, and intentional weight loss is generally not advised in pregnancy; the labels advise stopping before a planned pregnancy. Breastfeeding status is a conversation to have with your clinician.
- **Birth control:** some of these medicines (such as tirzepatide) can make *oral* contraception less reliable. If you could become pregnant, ask about backup or non-oral contraception.
- **Thyroid boxed warning:** this drug class carries an FDA boxed warning (thyroid C-cell tumours seen in rodents) and is not for people with a personal or family history of medullary thyroid cancer (MTC) or MEN 2.
- **Blood sugar:** combined with insulin or a sulfonylurea, these medicines can cause low blood sugar — those other medicines often need prescriber-managed adjustment, not self-adjustment.
- **Eating disorders:** an appetite-suppressing medicine warrants particular caution with a current or past eating disorder; disclose this to your clinician.
- **Surgery, sedation & endoscopy:** because these medicines slow stomach emptying, tell your surgical or anaesthesia team you take a GLP-1 well in advance — they'll decide with you whether to continue or hold it (more: https://thepeptideera.com/evidence/glp1-and-surgery).

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](/evidence/retatrutide-evidence)** (not yet filed with the FDA) and the GLP-1-plus-amylin combination **[CagriSema](/evidence/cagrisema-glp1-amylin)** (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](/evidence/glp1-brand-decoder) and [semaglutide vs tirzepatide](/evidence/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.

_Figure: Average weight loss in each drug's own main trial_
 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](/evidence/glp1-side-effects). 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](/evidence/the-evidence-ladder) — and which link of the supply chain — a specific product actually sits on.

## Sources (7)
1. Drucker DJ. Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metabolism. 2018;27(4):740–756. — https://doi.org/10.1016/j.cmet.2018.03.001 [REVIEW]
2. Ozempic (semaglutide) — FDA Prescribing Information via DailyMed (mechanism, indications) — https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=adec4fd2-6858-4c99-91d4-531f5f2a2d79 [LABEL]
3. Mounjaro (tirzepatide) — FDA Prescribing Information via DailyMed (dual GIP/GLP-1 receptor agonist) — https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d2d7da5d-ad07-4228-955f-cf7e355c8cc0 [LABEL]
4. Wegovy (semaglutide 2.4 mg) — FDA Prescribing Information via DailyMed (chronic weight management, cardiovascular indication) — https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ee06186f-2aa3-4990-a760-757579d8f77b [LABEL]
5. Zepbound (tirzepatide) — FDA Prescribing Information via DailyMed (chronic weight management; obstructive sleep apnea) — https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b [LABEL]
6. Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384:989–1002. — https://doi.org/10.1056/NEJMoa2032183 [RCT]
7. Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387:205–216. — https://doi.org/10.1056/NEJMoa2206038 [RCT]

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From The Peptide Era (https://thepeptideera.com) — evidence-graded, primary-sourced answers about GLP-1 medicines. Education, not medical advice. No doses, no sourcing.