"GLP-2" means tirzepatide, "GLP-3" means retatrutide. Except it does not.

Spend time in peptide forums, Reddit threads or social video and you will see a three-step ladder: GLP-1 for semaglutide, "GLP-2" for tirzepatide, "GLP-3" for retatrutide. The logic is roughly "one receptor, two receptors, three receptors". It is easy to remember. It is also pharmacologically wrong, and the error is not harmless, because GLP-2 already names something else.

What the three drugs actually target

Tirzepatide is a GIP and GLP-1 drug

The Zepbound label describes tirzepatide as a GIP receptor and GLP-1 receptor agonist. GIP stands for glucose-dependent insulinotropic polypeptide, a different gut peptide made mainly by K cells in the upper small intestine. Tirzepatide is a single molecule that activates both receptors. It does not act on the GLP-2 receptor. In SURMOUNT-1, the 15 mg dose produced a mean weight loss of 20.9% at 72 weeks vs 3.1% on placebo. The tirzepatide compound page and SURMOUNT trials page have the details.

If you want a short name that is accurate, "dual agonist" or "GIP/GLP-1" works.

Retatrutide is a GIP, GLP-1 and glucagon drug

Retatrutide, from Eli Lilly, activates three receptors: GIP, GLP-1 and glucagon. The third one is glucagon, the pancreatic peptide that raises blood sugar and increases energy use, not a "GLP-3". There is no peptide called GLP-3 in human physiology. In the phase 2 obesity trial published in the New England Journal of Medicine in 2023, 338 adults were randomised and the 12 mg dose produced a mean weight change of -24.2% at 48 weeks vs -2.1% on placebo. Retatrutide is not approved by any regulator. The retatrutide compound page and retatrutide trials page cover the phase 3 programme.

The accurate short name is "triple agonist" or "GIP/GLP-1/glucagon".

Semaglutide, for completeness

The bottom rung of the ladder is the only accurate one. Semaglutide is a GLP-1 receptor agonist. In STEP 1, semaglutide 2.4 mg weekly produced a mean weight loss of 14.9% at 68 weeks against 2.4% on placebo. The semaglutide page has the rest. Orforglipron (Foundayo) is also a GLP-1 receptor agonist, taken as a daily pill. Under the slang system it would also be "GLP-1", which shows how little the ladder says about the actual differences between drugs.

What the other receptors do

GIP and glucagon are the real additions. GIP, like GLP-1, is an incretin: a gut peptide that boosts insulin release after a meal. Glucagon comes from the pancreas and normally raises blood sugar between meals; in retatrutide its receptor activity is thought to increase energy use and liver fat clearance, balanced by the GLP-1 and GIP arms. If you want to compare these drugs, compare receptors, doses and trial results, not the number after "GLP".

What GLP-2 really is

GLP-2, glucagon-like peptide 2, is a gut peptide made by the same intestinal L cells as GLP-1, from the same precursor protein. Its job is to grow the intestinal lining and improve absorption. Its receptor was cloned in 1999, and cells carrying it responded to GLP-2 but not to GLP-1. It has no appetite effect in healthy people. The drugs that act on it, teduglutide, glepaglutide and apraglutide, are for short bowel syndrome, and their labels require colonoscopy monitoring because they make gut tissue grow. The what is GLP-2 page has the full story.

Why the mix-up matters

• Searching gives the wrong answers. Someone researching "GLP-2 side effects" for tirzepatide will find intestinal growth, polyps and colonoscopy schedules. Those belong to teduglutide, not tirzepatide.
• Labels and listings get confusing. Products described as "GLP-2" or "GLP-3" may refer to tirzepatide or retatrutide, and the name alone does not say which molecule is in the vial. The compounded glossary entry explains what the FDA allows and does not.
• Conversations with clinicians stall. A doctor hearing "I'm on GLP-2" will think of a short bowel drug. Naming the molecule and dose avoids that.
• It hides what is actually different. The real step from semaglutide to tirzepatide is the GIP receptor; the real step to retatrutide is the glucagon receptor. Those differences are where the side effect and efficacy differences come from.

Where the shorthand came from

The ladder seems to have spread as a way to rank drugs by how many receptors they hit, with each new receptor treated as a new "generation" of GLP. It took hold as tirzepatide reached the market and retatrutide drew attention in phase 2. It has no basis in the scientific or regulatory literature. Regulators, labels and trial papers use the drug names or the receptor names.

The ladder also breaks on its own terms. Survodutide and mazdutide hit two receptors, glucagon and GLP-1, but not GIP, so they would be a different "GLP-2" from tirzepatide. CagriSema pairs semaglutide with cagrilintide, an amylin analogue. Counting receptors tells you very little about which ones, and the which is what decides the effects.

Quick reference

If you see "GLP-1", it usually means semaglutide or liraglutide; see the GLP-1 hub. "GLP-2" online usually means tirzepatide; in medicine it means teduglutide and the short bowel drugs. "GLP-3" online means retatrutide; in medicine it means nothing.

What to log

Whatever people call it, log the molecule by its real name, with the dose in mg, the date and the injection site. Peptide AI's dose log, peptide library and vial tracker use the drug names, so the record stays unambiguous.