Reta peptide (retatrutide): what it is, how it works, what the trials show

Photograph: editorial illustration. It does not show retatrutide.

What is the reta peptide?

Reta is the nickname for retatrutide, a synthetic peptide developed by Eli Lilly and Company (https://www.lilly.com) under the code LY3437943. It is a chain of 39 amino acids built on the backbone of GIP, a natural gut hormone, and modified so that the same chain also fits two other receptors: the GLP-1 receptor, the target of semaglutide, and the glucagon receptor, which no approved obesity drug activates. A fatty diacid attached to the chain binds it to albumin in the blood and stretches its half-life to about six days, which is why it is injected once a week^[1][2].

People call it reta because retatrutide is long, and GLP-3 because it acts on three receptors where semaglutide acts on one and tirzepatide on two. Neither nickname is a product name. There is no approved retatrutide medicine anywhere, and every product sold under these names today is either laboratory reagent or an unapproved drug. We explain the nicknames in what is reta and GLP-3 peptide.

How does the reta peptide work?

Each receptor does a different job. GLP-1 receptor activation slows the stomach, lowers appetite and increases insulin only when glucose is high. GIP receptor activation reinforces the insulin effect and, combined with GLP-1, appears to improve tolerability and add weight loss. Glucagon receptor activation raises energy expenditure and pushes the liver to burn fat. The discovery paper in Cell Metabolism describes how the potency at each receptor was tuned, with GIP activity strongest and GLP-1 and glucagon activity set at a fraction of the native hormones, so that the incretin effects outweigh glucagon's tendency to raise blood sugar^[1].

The clearest fingerprint of the glucagon component is in the liver: in the phase 2 substudy of 98 people with fatty liver, liver fat fell by more than 80% in 24 weeks on the 8 mg and 12 mg doses while body weight had fallen by about 17%^[5]. The receptor-by-receptor account is in retatrutide mechanism of action.

What do the trials show?

Phase 2: the 48-week obesity trial

Published in the New England Journal of Medicine in 2023, the phase 2 obesity trial enrolled 338 adults with obesity or overweight and no diabetes. Mean weight change at 48 weeks was −8.7% on 1 mg, −17.1% on 4 mg, −22.8% on 8 mg and −24.2% on 12 mg, against −2.1% on placebo, and the curves had not flattened when the trial ended^[3].

A parallel trial in 281 people with type 2 diabetes, in The Lancet, found HbA1c reductions of about two percentage points on the higher doses at 24 weeks and weight change of −16.94% on 12 mg at 36 weeks^[4]. See retatrutide for type 2 diabetes.

Phase 3: the TRIUMPH program

Five phase 3 trials were registered under the name TRIUMPH from 2023, and four have reported. TRIUMPH-4, in 445 adults with knee osteoarthritis, reported −28.7% at 68 weeks on 12 mg in December 2025^[9]. TRIUMPH-1 (NCT05929066), the pivotal obesity trial in 2,339 adults, reported on 21 May 2026: 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; 45.3% of the 12 mg group lost at least 30% of their body weight, and an extension in people with a BMI of 35 or more reached −30.3% at 104 weeks^[6][7]. TRIUMPH-2 (diabetes) and TRIUMPH-3 (cardiovascular disease) reported −20.8% and −22.6% at 80 weeks on 12 mg in July 2026^[8].

For scale, semaglutide 2.4 mg produced −14.9% at 68 weeks in STEP 1^[12] and tirzepatide 15 mg produced −20.9% at 72 weeks in SURMOUNT-1^[11]. These are separate trials, not head-to-head comparisons; the caveats are in retatrutide vs Mounjaro. Every trial, with its registry number and dates, is on the clinical trials timeline.

What are the side effects of the reta peptide?

Gastrointestinal, dose-related, and concentrated during dose escalation. In TRIUMPH-1 the most common adverse events on 4, 9 and 12 mg versus placebo were nausea (28.6%, 38.4%, 42.4% versus 14.8%), diarrhoea (25.2%, 34.1%, 32.0% versus 13.5%) and constipation (23.8%, 25.9%, 26.1% versus 10.9%); discontinuation for adverse events was 4.1%, 6.9% and 11.3% versus 4.9%^[6]. The phase 2 trial also recorded a dose-dependent rise in heart rate that peaked around week 24 and then declined^[3]. The cardiovascular outcomes trial that will say whether that matters, TRIUMPH-Outcomes, runs to about 2029^[10]. The full record is in GLP-3 side effects.

Is the reta peptide approved?

No. Retatrutide has no marketing authorisation from the FDA, the EMA or any other regulator. In its 23 July 2026 announcement Lilly said it plans to submit a Biologics License Application to the FDA in the first quarter of 2027^[8]; a standard review takes about ten to twelve months from acceptance, so a decision before late 2027 is unlikely. Until then there is no approved dose, no label and no prescription route, and posts claiming otherwise are wrong, as we detail in retatrutide on Reddit and retatrutide FDA approval.

Where does research-grade retatrutide come from?

Because the sequence has been public since 2022, contract peptide manufacturers can synthesise retatrutide, and research-use-only suppliers sell it to laboratories as lyophilised powder with a certificate of analysis. That material is not the pharmaceutical-grade drug used in the trials, and it is not for human use. What separates a serious supplier from the rest is documentation: a certificate per lot naming the testing laboratory, with identity by mass spectrometry, purity by HPLC and net peptide content. We explain how to read one in how to read a retatrutide COA and describe the wider market in the retatrutide grey market.

For laboratories in the United States, one listing that publishes a batch certificate of analysis and ships domestically is the RP page at OXpeptides: See it at OXpeptides (research use only). This site has no commercial relationship with any supplier and earns nothing from that link.

All articles on this site

• 
  What is reta

  What is reta? The peptide behind the nickname, explained

  Reta is short for retatrutide, Eli Lilly's weekly triple agonist peptide (GIP, GLP-1 and glucagon receptors).

• 
  GLP-3 peptide

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

  GLP-3 is not a hormone.

• 
  Retatrutide mechanism of action

  Retatrutide mechanism of action: how one peptide works on three receptors

  How retatrutide activates the GIP, GLP-1 and glucagon receptors, what each receptor contributes to weight loss and glucose control, and what the discovery paper in Cell Metabolism reported..

• 
  Retatrutide clinical trials

  Retatrutide clinical trials: the TRIUMPH timeline and every result so far

  Every retatrutide trial with its ClinicalTrials.gov number, dates, size and result: phase 1, phase 2, and the TRIUMPH-1 to TRIUMPH-4 phase 3 read-outs of 2025 and 2026, plus the outcomes trial running to 2029..

• 
  Retatrutide FDA approval

  Retatrutide FDA approval: where it stands in September 2026 and what comes next

  Retatrutide is not FDA approved.

• 
  Retatrutide vs Mounjaro

  Retatrutide vs Mounjaro: what the trials of the two Lilly peptides show

  Retatrutide and tirzepatide (Mounjaro, Zepbound) compared on receptors, weight loss, HbA1c, side effects and approval status, using SURMOUNT, SURPASS, phase 2 and TRIUMPH data..

• 
  GLP-3 side effects

  GLP-3 side effects: what the retatrutide trials actually report

  The side effects of the so-called GLP-3 peptide, retatrutide, taken from the phase 2 publications and the phase 3 TRIUMPH topline results: nausea, diarrhoea, constipation, heart rate, discontinuation rates and what remains unknown..

• 
  Retatrutide Reddit

  Retatrutide on Reddit: seven things people get wrong, checked against the trials

  The most repeated claims about retatrutide on Reddit, each one checked against the phase 2 publications and the phase 3 TRIUMPH results: dosing, speed of weight loss, side effects, approval, and what a vendor COA proves..

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  Retatrutide COA

  Retatrutide COA: how to read a certificate of analysis, line by line

  What a retatrutide certificate of analysis must contain (identity by mass spectrometry, HPLC purity, net content, lot, lab), how to check it, and the signs of a decorative or reused COA..

• 
  Retatrutide grey market

  The retatrutide grey market: what is actually being sold, and what to check

  Retatrutide is unapproved, yet it is sold online as a research chemical, through compounding-style sellers and through social media.

• 
  Retatrutide fatty liver

  Retatrutide and fatty liver: what the phase 2 MASLD substudy found

  In a phase 2a substudy of 98 adults with MASLD, retatrutide 8 mg and 12 mg cut liver fat by more than 80% in 24 weeks, and most participants reached normal liver fat.

• 
  Retatrutide diabetes

  Retatrutide for type 2 diabetes: what the phase 2 trial and TRIUMPH-2 found

  Retatrutide in type 2 diabetes: HbA1c reductions of about two points at 24 weeks in phase 2, −16.94% weight at 36 weeks on 12 mg, and −20.8% at 80 weeks with HbA1c −1.5 in TRIUMPH-2.