GLP-3 peptide: there is no such hormone, and what people mean when they say it

By Retatrutide Risk editorial team. Published September 21, 2026. Sources at the end of the page.

"GLP-3" appears on vendor sites, in forum threads and in search boxes, and almost always refers to retatrutide. The name is convenient and wrong. This page explains why, what the actual biology is, and what the trials say about the compound the name points to.

Two glucagon-like peptides exist, not three

The human proglucagon gene is processed differently in different tissues. In the pancreas it yields glucagon. In the intestine and brain it yields glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). GLP-1 stimulates insulin release after meals, slows gastric emptying and reduces appetite; its receptor is the target of semaglutide, liraglutide, dulaglutide and, together with the GIP receptor, tirzepatide. GLP-2 acts on the intestinal lining and is the target of drugs for short bowel syndrome. There is no GLP-3 in the proglucagon sequence, no GLP-3 receptor, and no GLP-3 in any pharmacology reference.

The term was coined by analogy: if a drug that hits the GLP-1 receptor is "a GLP-1", and tirzepatide hits two receptors, then a drug that hits three must be "GLP-3". The analogy fails because the three receptors retatrutide activates are GIP, GLP-1 and glucagon. Only one of them is a GLP receptor.

What "GLP-3" actually refers to

Retatrutide is a single synthetic peptide developed by Eli Lilly under the code LY3437943. The discovery paper describes it as a triple agonist at the glucagon receptor, the GIP receptor and the GLP-1 receptor, with balanced glucagon and GLP-1 receptor activity and more GIP receptor activity in vitro. It is administered as a once-weekly subcutaneous injection, and its half-life is approximately 6 days.

Why the third receptor is glucagon, and why that matters for risk

Glucagon is the hormone that raises blood glucose between meals by telling the liver to release stored glucose. Activating its receptor deliberately in a drug for obesity and diabetes seems counterintuitive. The rationale in the discovery paper is that glucagon receptor activation increases energy expenditure and reduces liver fat, and that the glucose-raising effect is offset by the insulin-promoting effects of the GIP and GLP-1 components. In mice, the paper reports, weight loss from reduced calorie intake was augmented by glucagon-mediated increases in energy expenditure.

The clinical evidence supports the offset. In the 36-week phase 2 trial in type 2 diabetes, HbA1c fell by 1.39% to 2.02% at doses of 4 mg and above, compared with a 0.01% fall on placebo and a 1.41% fall on dulaglutide 1.5 mg. In the liver substudy, liver fat fell by 81.4% (8 mg) and 82.4% (12 mg) in 24 weeks, versus a 0.3% rise on placebo.

The glucagon component is also the reason retatrutide's safety profile cannot simply be assumed to match the GLP-1 drugs. The phase 2 obesity trial reported dose-dependent increases in heart rate that peaked at 24 weeks. Whether the glucagon receptor contributes to that effect, and to the dysesthesia reported in phase 3, has not been established in any published source.

"GLP-3 side effects": what the trials report

Because "GLP-3" is retatrutide, its side effects are retatrutide's. The published and announced data are summarized here; the side effects article has the full tables.

The name as a marketing signal

The "GLP-3" label is used mainly by sellers. It positions retatrutide as the next step in a familiar series, which makes an investigational compound sound like an established product category. Two facts cut against that framing. Retatrutide is not approved anywhere; Lilly has said it plans to file with the FDA in the first quarter of 2027. And the product sold under the name is a research chemical whose relationship to the trial drug is limited to the intended sequence; purity and concentration are whatever the seller achieves. A listing that says "GLP-3" is not a red flag by itself, but a listing that treats "GLP-3" as an approved class is misrepresenting the compound's status.

What the trials measured, in brief

The compound the name points to has been studied in a 48-week phase 2 obesity trial of 338 adults, where mean weight change at 12 mg was -24.2% versus -2.1% on placebo, and in a 36-week phase 2 trial in type 2 diabetes of 281 adults, where the 12 mg dose reduced HbA1c by 2.02% and weight by 16.94%. The phase 3 TRIUMPH-1 trial, reported by the company in May 2026 and not yet peer-reviewed, found -28.3% at 12 mg over 80 weeks in 2,339 participants. The results article sets out every figure by dose and time point, and the dosage article explains the escalation schedules the trials used and why they are not a recommendation.

Terms that are used correctly

• Retatrutide: the international nonproprietary name.
• LY3437943: Lilly's development code, used in the phase 1 papers and every ClinicalTrials.gov record.
• Triple agonist, triple hormone receptor agonist: the description used in the NEJM and Nature Medicine titles.
• GIP/GLP-1/glucagon receptor agonist: the description used in the Lancet titles.

Any of these will find the primary literature. "GLP-3" will find vendors.

Related reading

The reta peptide article covers the molecule and the trial timeline. The comparison with tirzepatide sets the dual agonist and the triple agonist side by side. The FDA approval article covers the regulatory status.