What are peptides? GLP-1, “GLP-2” and “GLP-3” explained simply

You’ve heard of Ozempic and Mounjaro. Maybe someone at the gym told you about “GLP-3”. This article explains what all of it means without assuming any biology. It also clears up a common mix-up: two of those three names don’t mean what most people think.

Part 1: What is a peptide?

Start with amino acids. There are 20 common kinds, and your body joins them together like beads on a string. The order of the beads decides what the string does.

The order matters the way letter order matters in words. “Listen” and “silent” use the exact same six letters. Put them in a different order and you get a different word. Amino acids work the same way.

The only difference between a peptide and a protein is length:

There’s no strict cut-off, but scientists usually call anything under about 50 amino acids a peptide and anything longer a protein.

Peptides as messages

Many peptides in your body are hormones, which are messages carried through your blood. A cell in one organ releases the message, and a cell somewhere else picks it up.

To pick it up, the receiving cell needs a receptor. Think of a receptor as a doorbell that only rings for one specific visitor. The GLP-1 receptor rings when GLP-1 arrives and ignores everything else. Once it rings, the cell does its job: release insulin, send a “full” signal, and so on.

Almost every drug in this article works the same way. It’s a man-made peptide built to ring one or more of these doorbells, and to keep ringing them for much longer than the real hormone does.

Why most peptide drugs are injections

Your stomach can’t tell a peptide drug from a piece of chicken or a bowl of dal. Both are chains of amino acids, and your digestive system cuts both back into single beads. Swallow most peptides and they get digested before they can do anything.

You can see the problem in semaglutide, which comes both ways. The weekly injection (Wegovy) is 2.4 mg once a week. The tablet (Rybelsus) needs an absorption helper, and even then it is 7 to 14 mg every day. That’s up to 98 mg a week to do the job of 2.4 mg injected, roughly 40 times more drug, because most of it is lost in the gut.

The same logic applies to the collagen peptide powders sold as supplements. Most of what you swallow is broken down into amino acids, the same as any other protein.

Part 2: GLP-1, the “food has arrived” message

GLP-1 stands for glucagon-like peptide-1. The name is confusing, so here’s where it comes from.

Your body has one gene that makes a long parent molecule called proglucagon. Different organs cut that molecule up in different places, a bit like cutting different shapes from the same sheet of paper. Your pancreas cuts out glucagon. Your gut cuts out GLP-1 and GLP-2. They’re called “glucagon-like” because they come from the same sheet and look a bit like glucagon. That’s all the name means.

What GLP-1 does

Say you sit down to a plate of rice, dal and sabzi. Within minutes, cells lining your small intestine detect the food and release GLP-1. It does three things:

• It tells your pancreas to release insulin, but only if your blood sugar is going up. This detail matters. If your sugar is already normal, GLP-1 mostly backs off. That’s why GLP-1 drugs on their own rarely push blood sugar dangerously low, unlike insulin.
• It slows your stomach down. Food leaves the stomach more slowly, so you stay full longer and sugar enters your blood more gradually.
• It tells your brain you’ve eaten. GLP-1 receptors in the brain reduce hunger. Many people on these drugs say the constant background thinking about food goes quiet.

The two-minute problem, and a lizard

Natural GLP-1 has a big weakness: an enzyme in your blood called DPP-4 destroys it in about two minutes. That works fine for a signal that only needs to last through a meal. As a medicine it’s useless, because you’d need an injection every few minutes.

The fix came from an unlikely place. In the early 1990s, researcher John Eng studied the venom of the Gila monster, a large lizard from the American Southwest. He found a peptide, exendin-4, that rings the GLP-1 doorbell but resists DPP-4. A lab-made copy became exenatide, the first GLP-1 drug, approved in 2005. It still had to be injected twice a day.

Later drugs lasted longer. Liraglutide (2010) needed one injection a day. Semaglutide (2017) needs one a week. The trick with semaglutide was attaching a small fatty tail that makes it stick to albumin, the most common protein in your blood. Stuck to albumin, the kidneys don’t filter it out and enzymes struggle to reach it. It lasts about a week instead of two minutes.

How well it works

In the main weight-loss trial (STEP-1), people on semaglutide 2.4 mg lost about 15% of their body weight on average over 68 weeks. People on placebo lost about 2.4%. For someone who weighs 90 kg, 15% is roughly 13.5 kg.

Part 3: The real GLP-2, which has nothing to do with weight loss

Remember the sheet of paper? The same gut cells that release GLP-1 release GLP-2 at the same moment. GLP-2 has a completely different job.

The inside of your intestine is covered in a thin layer of cells that absorb nutrients from food. That lining wears out fast and replaces itself every three to five days. GLP-2 keeps that lining growing and healthy, so your gut can absorb what you eat.

There is a real GLP-2 medicine: teduglutide (sold as Gattex or Revestive). It’s used for short bowel syndrome, a condition where people have lost a large part of their intestine to surgery or disease and can’t absorb enough food. Teduglutide helps what’s left absorb more. That’s close to the opposite of a weight-loss drug.

So when someone calls Mounjaro “GLP-2”, they’ve borrowed the name of a real hormone that does something unrelated. It’s worth knowing in case you ever search for GLP-2 and wonder why the medical results are about gut surgery.

Part 4: Where the “GLP-2” and “GLP-3” nicknames come from

The nicknames count how many doorbells a drug rings. One doorbell gets called GLP-1, two get called “GLP-2”, three get called “GLP-3”. It’s easy to remember, but scientifically wrong. Here’s what each drug actually does:

Part 5: Tirzepatide, two doorbells

Tirzepatide rings the GLP-1 doorbell plus a second one called GIP (glucose-dependent insulinotropic polypeptide). GIP is another “food has arrived” message. It comes from cells a little higher up in the small intestine and also tells the pancreas to release insulin when sugar is high.

Tirzepatide is one molecule, not two drugs mixed together. Its 39 amino acids are mostly based on GIP’s sequence, adjusted so it also rings the GLP-1 doorbell.

Why add GIP? Honestly, scientists are still working that out. GIP on its own doesn’t cause much weight loss, yet adding it to GLP-1 clearly does more than GLP-1 alone. One theory is that GIP acts on parts of the brain that reduce nausea, so the body tolerates the combination better. The results are clearer than the explanation.

In the main trial (SURMOUNT-1), people on the highest dose lost about 21% of their body weight on average over 72 weeks. For a 90 kg person that’s about 19 kg. In a 2025 head-to-head trial (SURMOUNT-5), tirzepatide averaged about 20% weight loss against about 14% for semaglutide.

Part 6: Retatrutide, three doorbells

Retatrutide rings GLP-1, GIP and a third doorbell: glucagon. That’s surprising at first, because glucagon raises blood sugar. It’s the message your pancreas sends when you haven’t eaten for a while, telling your liver to release stored energy.

Why add a sugar-raising message to a diabetes and weight drug? Because glucagon also pushes the liver to burn fat and makes the body spend more energy. The GLP-1 and GIP parts keep blood sugar under control while the glucagon part increases energy use.

Picture a bucket of water. GLP-1 and GIP turn down the tap (you eat less). Glucagon opens the drain a bit wider (you burn more). Doing both empties the bucket faster than either one alone.

In the phase 2 trial published in 2023, people on the highest dose lost about 24% of their body weight on average over 48 weeks. That’s about 22 kg for a 90 kg person, in less time than the other trials. Side effects were similar to the other drugs (mostly nausea and other stomach problems), plus a dose-related rise in heart rate. Larger phase 3 trials are under way. Until regulators review the full results, retatrutide is not an approved medicine anywhere in the world.

Part 7: Why the biggest number isn’t automatically “best”

It’s tempting to line up 15%, 21% and 24% and pick the biggest. But:

• They come from different trials. Different people, different lengths, different doses. Only head-to-head trials compare drugs fairly, and the only big one so far is tirzepatide vs semaglutide.
• Averages hide a wide range. In every trial, some people lost very little and some lost far more than average.
• Approved means studied for safety. Semaglutide and tirzepatide have years of data from millions of patients. Retatrutide has much less.
• None of them is a one-off fix. The weight-regain data applies to the whole class.

A note about research peptides

Athenex Labs supplies some of the molecules in this article, such as retatrutide and tirzepatide, as research compounds for laboratory work. Research compounds are not medicines. They aren’t made or tested for injection into people, they don’t come with dosing guidance, and nobody is monitoring the person using them.

If you’re reading this because you’re thinking about these drugs for your own weight or blood sugar, see a doctor. Semaglutide and tirzepatide are approved and prescribed in India, and a doctor can check whether they suit you and watch for side effects.

Common questions

Is Mounjaro a GLP-2?

No. Mounjaro (tirzepatide) rings two doorbells, GLP-1 and GIP, which is where the nickname comes from. The real GLP-2 is a gut-lining hormone with no role in weight loss.

Is there a hormone called GLP-3?

No. “GLP-3” is slang for retatrutide, which acts on GLP-1, GIP and glucagon receptors. Your body doesn’t make anything called GLP-3.

Are peptides the same as steroids?

No. Steroids, like testosterone or cortisone, are built from cholesterol and have a ring-shaped structure. Peptides are chains of amino acids. They’re different types of molecule that happen to show up in the same gym conversations.

Why can’t I just take GLP-1 as a pill?

You can in one case: oral semaglutide (Rybelsus). But because your stomach digests peptides like food, it needs an absorption helper and a much bigger dose. For most peptides, injection is the only practical way in.

Sources

• Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med 2021;384:989–1002.
• Wilding JPH et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide (STEP 1 extension). Diabetes Obes Metab 2022;24:1553–1564.
• Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 2022;387:205–216.
• Aronne LJ et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med 2025.
• Jastreboff AM et al. Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial. N Engl J Med 2023;389:514–526.
• Eng J et al. Isolation and characterization of exendin-4, an exendin-3 analogue, from Heloderma suspectum venom. J Biol Chem 1992;267:7402–7405.
• Jeppesen PB et al. Teduglutide reduces need for parenteral support among patients with short bowel syndrome with intestinal failure. Gastroenterology 2012;143:1473–1481.
• Drucker DJ. Mechanisms of action and therapeutic application of glucagon-like peptide-1. Cell Metab 2018;27:740–756.

This article is for general education only and is not medical advice.