GLP-3 peptide: what the name means, and what retatrutide actually is

Photograph: editorial illustration for this article. It does not show retatrutide.

Why do people say GLP-3?

The label started as a joke and became a search term. Semaglutide made GLP-1 a household word. Tirzepatide, which adds GIP, was sometimes called GLP-2 by people who had never heard of the real GLP-2 (a gut hormone unrelated to weight). When retatrutide added a third receptor, the same logic produced GLP-3. It is catchy, wrong as biology, and now searched 12,100 times a month in the United States according to Google Ads data we pulled in September 2026.

So when a forum post, a vendor page or a video mentions the GLP-3 peptide, it means retatrutide, Lilly's compound LY3437943. The only honest thing the name tells you is the number of receptors involved.

One, two, three receptors

The receptor count matters because each receptor brings a distinct effect. GLP-1 receptor activation slows stomach emptying, lowers appetite and increases insulin release when glucose is high. GIP receptor activation works on fat tissue and the pancreas, and in combination with GLP-1 it appears to amplify weight loss while easing nausea. Glucagon receptor activation raises energy expenditure and drives the liver to oxidise fat, but on its own it also raises blood glucose^[1].

Retatrutide was engineered as a single 39-amino-acid peptide on a GIP backbone with a fatty diacid chain for weekly dosing. In cell assays it activated the GIP receptor more strongly than native GIP and activated the GLP-1 and glucagon receptors at a fraction of the native hormones' potency, a balance chosen deliberately so that the incretin effects offset the glucose-raising effect of glucagon^[1]. Our mechanism article goes through the pharmacology receptor by receptor.

What does the glucagon receptor add?

Two things, according to the discovery paper and the phase 2 data. First, more weight loss than the incretin receptors alone seem to deliver. Second, a large reduction in liver fat: in the phase 2 liver substudy, participants on 8 mg or 12 mg lost more than 80% of their liver fat by 24 weeks, which we cover in retatrutide and fatty liver. The price is a higher resting heart rate in the first months and a dose-related run of gastrointestinal effects, described in GLP-3 side effects.

What are the trial numbers behind the GLP-3 label?

The 48-week phase 2 obesity trial (338 adults, no diabetes) reported mean weight change of −24.2% on 12 mg, −22.8% on 8 mg, −17.1% on 4 mg and −8.7% on 1 mg, against −2.1% on placebo^[2]. For scale, semaglutide 2.4 mg produced −14.9% at 68 weeks in its pivotal STEP 1 trial^[3], and tirzepatide 15 mg produced −20.9% at 72 weeks in SURMOUNT-1^[4]. These are separate trials with different durations and populations, so the comparison is indicative, not a head-to-head result.

Phase 3 moved the number higher. TRIUMPH-1 (NCT05929066), with 2,339 participants, reported mean weight change at 80 weeks of −19.0% on 4 mg, −25.9% on 9 mg and −28.3% on 12 mg, against −2.2% on placebo^[5][6]. Note that phase 3 used a 9 mg middle dose rather than the 8 mg of phase 2.

Are there other GLP-3 peptides?

Several companies have triple agonists in early development, and vendors sometimes attach the GLP-3 label to any of them. None of those has published phase 3 results, and none is the compound people are actually searching for. If a product is called GLP-3 without naming retatrutide or LY3437943, treat the label as marketing. If it does name retatrutide, the only way to know what is in the container is a certificate of analysis, which we explain in how to read a retatrutide COA.

What is the status of the GLP-3 peptide in 2026?

Retatrutide is investigational. Four TRIUMPH phase 3 trials have completed and reported topline results between December 2025 and July 2026, and Lilly has stated that it plans to submit a Biologics License Application to the FDA in the first quarter of 2027^[7]. Until a regulator acts, there is no approved GLP-3 product, no label and no prescription route.