"GLP-1 vs GLP-3" reads like a comparison between two hormones. It is not. GLP-1 is a human hormone and the name of an approved drug class; there is no human GLP-3. The phrase comes from how some online sellers name an unapproved drug. This page sets out what the gene makes, where a real GLP-3 does exist, and what the name means on a product page, from the sequence database, the literature and FDA warning letters, as of 27 September 2026.
For the real second hormone in this family, see GLP-1 vs GLP-2; for the drug "GLP-3" usually refers to, retatrutide.
What the human gene makes
GLP-1 and GLP-2 are cut from one precursor protein, proglucagon. UniProt's curated entry for the human protein (P01275) lists the pieces:
| Peptide | Position in proglucagon | What it is known for |
|---|---|---|
| Glicentin | 21–89 | gut hormone, largely inactive precursor |
| Oxyntomodulin | 53–89 | acts on both GLP-1 and glucagon receptors |
| Glucagon | 53–81 | raises blood sugar; the "third" target in retatrutide |
| GLP-1 (7–37 / 7–36) | 98–128 | insulin release, appetite — the basis of semaglutide |
| GLP-2 | 146–178 | growth of the gut lining — the basis of teduglutide |
That is the whole list. No third glucagon-like peptide is cut from the human protein.
Where a real GLP-3 does exist
In sharks. A 2018 survey of fish genomes (Irwin and Mojsov, General and Comparative Endocrinology) found that cartilaginous fish carry a proglucagon gene encoding "a novel GLP-3 peptide in addition to glucagon, GLP-1, and GLP-2". In 2024 a Canadian team perfused the liver of the Pacific spiny dogfish with it and found that it switched the liver from producing ketones to consuming them, with no effect on glucose handling.
That is the entire scientific literature on GLP-3 as a hormone. On 27 September 2026, PubMed held five records for the exact phrase "glucagon-like peptide-3"; only the dogfish study names a GLP-3 peptide in its title, and none concerns a human medicine. (The unhyphenated phrase returns thousands of hits only because the search engine splits it into "glucagon-like peptide" and "3".)
What "GLP-3" means when it is being sold
FDA warning letters to online peptide sellers quote the product names exactly as the sellers listed them. Three examples:
| Seller (FDA letter date) | Product names quoted by FDA |
|---|---|
| Lovega LLC, dba Pink Pony Peptides (31 March 2026) | "GLP-2 TZ", "GLP-3 RT" |
| Peptide Partners LLC (24 August 2026) | "GLP-1 S (Semaglutide)", "GLP-2 T (Tirzepatide)", "GLP-3 Reta (Retatrutide)" |
| NuScience Peptides LLC (24 August 2026) | "GLP-1 Sema Research Peptide", "GLP-2 Tirz Peptide", "GLP-3 RT (Retatrutide) Research Peptide" |
The numbering is a count of receptors, not a hormone name. Semaglutide acts on one receptor (GLP-1), tirzepatide on two (GIP and GLP-1), retatrutide on three (GIP, GLP-1 and glucagon). One seller's copy, quoted by the FDA, calls retatrutide "three times the fat-fighting horsepower in one clean, clinical ride". The scheme is not universal — another March 2026 letter, to Gram Peptides, quotes retatrutide sold as "GLP-1-R peptide" and tirzepatide as "GLP-2 peptide" — which is itself the point: the name is marketing, and it varies by seller.
It also collides with real biology. "GLP-2" is already a human hormone, and the approved GLP-2 drug, teduglutide, grows the gut lining in people with short bowel syndrome — nothing to do with tirzepatide. Someone who reads "GLP-2" on a vial and looks it up will find the wrong drug.
In each of these letters the FDA treats the products as unapproved new drugs, and says that a "research use only" or "not for human consumption" label does not change that when the product is sold with injection supplies and weight-loss claims. The wider pattern of these letters is tracked on Peptifact's FDA warning-letter page.
GLP-1 drugs vs retatrutide, side by side
| Approved GLP-1 drugs (e.g. Wegovy, Ozempic) | Retatrutide ("GLP-3" online) | |
|---|---|---|
| Receptors targeted | GLP-1 (tirzepatide adds GIP) | GIP, GLP-1 and glucagon |
| FDA status | approved, with labels | not approved for any use |
| Legitimate source | prescription, licensed pharmacy | clinical trials only |
| What a "GLP-3" vial contains | — | unverified; no label, no FDA oversight |
Retatrutide's trial results, announced filing plans and the doses its trials used are on our retatrutide page, and how it compares with the approved dual agonist is on retatrutide vs tirzepatide. The approved GLP-1 drugs are listed on which drugs are GLP-1s.
What this page does not say
It does not say what is inside any product sold as "GLP-3" — nobody outside the seller can, and the letters quoted here cite the sellers' websites, not tests of what was in the vials. It does not compare the safety of retatrutide with approved drugs beyond regulatory status; the trial record belongs on the retatrutide page. And it does not tell anyone what to use: treatment decisions sit with a prescriber, and FDA's own consumer page, FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss, sets out the agency's position on products bought outside that route.
Sources
- UniProt P01275, Pro-glucagon (Homo sapiens), peptide features, read 27 September 2026.
- Irwin DM, Mojsov S. Diversification of the functions of proglucagon and glucagon receptor genes in fish. Gen Comp Endocrinol 2018;261:148–165. doi:10.1016/j.ygcen.2018.03.003.
- Weinrauch AM et al. The chondrichthyan glucagon-like peptide 3 regulates hepatic ketone metabolism in the Pacific spiny dogfish Squalus suckleyi. Gen Comp Endocrinol 2024;350:114470. doi:10.1016/j.ygcen.2024.114470.
- PubMed searches "glucagon-like peptide-3" (5 records) and "GLP-3" (11 records, none about a human drug), 27 September 2026.
- FDA warning letters: Lovega LLC dba Pink Pony Peptides (MARCS-CMS 721088, 31 March 2026); Gram Peptides (721806, 31 March 2026); Peptide Partners LLC (735063, 24 August 2026); NuScience Peptides LLC (733652, 24 August 2026).