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GLP-1 vs GLP-2 vs GLP-3: What the Names Actually Mean

October 20, 2026#glp-1 #retatrutide #lab-reference

Search for "GLP-3" and you will find it described as the next generation after GLP-1 and GLP-2, as if the three were siblings in one family. They are not. Two of those names are real peptide hormones with very different jobs, and the third does not exist in biology at all. The numbering is a nickname scheme that grew up online, and it now causes real confusion about what a compound is, which receptors it targets, and how far along it is in development.

This guide sorts it out: where the names came from, which receptors each compound actually acts on, and where each one stands with the FDA as of October 2026.

The short answer

GLP-1 is a real hormone. "GLP-1" compounds such as semaglutide activate its receptor.
"GLP-2" is used online as a nickname for tirzepatide, which activates two receptors (GLP-1 and GIP). The real GLP-2 is a separate gut hormone, and tirzepatide does not act on its receptor.
"GLP-3" is a nickname for retatrutide, an investigational triple agonist (GLP-1, GIP and glucagon receptors). There is no GLP-3 hormone.

Where GLP-1 and GLP-2 come from

GLP stands for glucagon-like peptide. GLP-1 and GLP-2 are both cut from the same precursor protein, proglucagon, which is made by specialized L-cells in the intestinal lining. GLP-2 is a 33-amino-acid peptide derived from the proglucagon gene (WJGPT, 2026).

Sharing a precursor is where the family resemblance ends. The two peptides bind different receptors:

GLP-1 is an incretin hormone, and its receptor is one of the most heavily researched targets in metabolic science.
GLP-2 is intestinotrophic, meaning it acts on the gut itself. Its receptor signals adaptation of the intestinal lining and absorption of nutrients and fluid (Superpower).

That is the first thing the online numbering gets wrong. "GLP-2" in the hormone sense has nothing to do with metabolic research.

GLP-1 receptor agonists: one target

The compounds people mean by "GLP-1" activate a single receptor, the GLP-1 receptor. The best known is semaglutide (sold as Ozempic and Wegovy), along with the older liraglutide (Victoza, Saxenda).

Most GLP-1 receptor agonists are peptides. That changed in 2026: on April 1 the FDA approved orforglipron (Foundayo), a once-daily tablet from Eli Lilly. It is a small molecule rather than a peptide, so it reaches the same receptor without being a peptide at all (Drugs.com).

Status: semaglutide, liraglutide and orforglipron are FDA-approved prescription drugs.

"GLP-2": tirzepatide and the dual agonists

The online logic goes like this: semaglutide hits one receptor, so it is "GLP-1"; tirzepatide hits two, so it is "GLP-2." It is a convenient count, but the label is wrong. Tirzepatide (Mounjaro, Zepbound) is a dual GIP/GLP-1 receptor agonist. It activates the GLP-1 receptor plus the receptor for GIP, a second incretin hormone. It does not act on the GLP-2 receptor (Noom).

Tirzepatide is not the only dual agonist. Survodutide and mazdutide pair the GLP-1 receptor with the glucagon receptor instead of GIP (Peptide Dossier). "Two receptors" can mean different combinations.

Status: tirzepatide is FDA-approved. Survodutide and mazdutide are investigational.

The real GLP-2: a gut peptide

Because the nickname has taken over search results, the actual GLP-2 class rarely gets mentioned. It exists, and it is researched for intestinal failure rather than metabolism.

Native GLP-2 is cleared within minutes, so researchers built longer-lasting analogs (JPET, via PubMed):

Teduglutide (Gattex, Revestive) has been approved for short bowel syndrome since 2012 and is the only approved GLP-2 receptor agonist.
Glepaglutide and apraglutide are longer-acting analogs in development for the same condition. Both remain investigational (PMC review).

None of this overlaps with tirzepatide.

"GLP-3": retatrutide, the triple agonist

There is no GLP-3 hormone. "GLP-3," "GLP-3 RT" and "RT-3" are informal names for retatrutide (development code LY3437943), Eli Lilly's investigational once-weekly compound that activates three receptors at once: GLP-1, GIP and glucagon (Drugs.com).

The glucagon arm is the new part. Tirzepatide already combines GLP-1 and GIP; retatrutide adds the glucagon receptor on top. How much that third receptor contributes to retatrutide's overall profile is one of the main open questions researchers are asking about it.

Where retatrutide stands in development

May 21, 2026: Lilly reported positive topline results from TRIUMPH-1, an 80-week Phase 3 program readout involving 2,339 participants (The Pharmaceutical Journal).
July 23, 2026: Lilly reported positive topline results from TRIUMPH-2 and TRIUMPH-3, and said it now has the data package to support global regulatory submissions (Eli Lilly via BioSpace).
Next: Lilly plans to file with the FDA in the first quarter of 2027 (SingleCare). FDA review usually takes 10 to 12 months after filing (Drugs.com).

Two caveats matter for anyone reading the coverage. First, these are company-reported topline results; full data is still due in peer-reviewed journals. Second, no head-to-head comparison of retatrutide against semaglutide or tirzepatide has been published yet (The Rx Index), so comparisons drawn across separate programs should be read cautiously.

Status: not FDA-approved, and not available as a prescription drug.

Side-by-side comparison

GLP-1 — GLP-1 receptor agonist. Receptor: GLP-1. Examples: semaglutide, liraglutide, orforglipron. US status: approved.
"GLP-2" (nickname) — dual incretin agonist. Receptors: GLP-1 + GIP. Example: tirzepatide. US status: approved.
Dual GLP-1/glucagon agonists — Receptors: GLP-1 + glucagon. Examples: survodutide, mazdutide. US status: investigational.
GLP-2 (the real peptide) — intestinotrophic analog. Receptor: GLP-2. Examples: teduglutide (approved); glepaglutide, apraglutide (investigational).
"GLP-3" / RT-3 (nickname) — triple agonist. Receptors: GLP-1 + GIP + glucagon. Example: retatrutide (LY3437943). US status: investigational, Phase 3 readouts complete, filing planned for 2027.

Why the naming matters in the lab

Nicknames are fine in conversation, but they are a poor way to identify a research compound. "GLP-2" can point to tirzepatide or to teduglutide, two molecules with nothing in common. "GLP-3" names a receptor count, not a structure. For any research material, the reliable identifier is the compound's nonproprietary name or development code (retatrutide or LY3437943, for example), confirmed by analytical testing. Our research peptide library lists the CAS number, sequence and identifiers for each compound.

That is why identity testing matters as much as purity (see how to read a peptide COA). Every Central Coast Peptides lot is checked by an independent ISO/IEC 17025-accredited laboratory, using HPLC for purity and LC-MS to confirm molecular identity against a reference standard. See our Quality Commitment, and our guide to storing lyophilized peptides once material arrives.

A note on the regulatory picture

Approved GLP-1 drugs are under close FDA attention. The shortage-era allowance for compounding tirzepatide and semaglutide ended in 2025, and on May 1, 2026 the FDA proposed keeping semaglutide, tirzepatide and liraglutide off the list of bulk substances outsourcing facilities may compound from (Science-Based Medicine). In early 2026 the agency also sent dozens of warning letters to telehealth companies over marketing that implied compounded products were the same as approved brands (GLP1Clinics). Retatrutide sits outside all of this because there is no approved product to compound from yet.

Frequently asked questions

Is there a GLP-3 hormone?

No. The body makes GLP-1 and GLP-2, both cut from proglucagon. "GLP-3" is an informal nickname for retatrutide, which activates three receptors: GLP-1, GIP and glucagon.

Is tirzepatide a GLP-2?

No. Tirzepatide is a dual GIP/GLP-1 receptor agonist. It gets called "GLP-2" because it targets two receptors, but it does not act on the GLP-2 receptor.

What does the real GLP-2 do?

GLP-2 acts on the gut, signaling adaptation of the intestinal lining. Its approved analog, teduglutide, is indicated for short bowel syndrome.

Is retatrutide FDA-approved?

No. As of October 2026 it is investigational. Lilly reported Phase 3 topline results in May and July 2026 and plans to file with the FDA in the first quarter of 2027.

What is the difference between a GLP-1, a dual agonist and a triple agonist?

A GLP-1 receptor agonist activates one receptor. A dual agonist such as tirzepatide activates the GLP-1 and GIP receptors. A triple agonist such as retatrutide activates the GLP-1, GIP and glucagon receptors.

Sources

All Central Coast Peptides products are sold strictly for laboratory research use. They are not drugs, foods, dietary supplements, or cosmetics, have not been evaluated by the FDA, and are not for personal or veterinary use. This article summarizes published research and regulatory status for educational purposes. It is not medical advice and does not describe any use of our products.

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