Reta peptide: what retatrutide is, how it works and what the evidence says

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

"Reta" is the name people use for retatrutide when they are typing quickly, and the search volume for it dwarfs every other retatrutide query. The short name hides a fairly precise object: a synthetic peptide engineered by Eli Lilly, code-named LY3437943, that is designed to act on three hormone receptors at once. This page explains what the molecule is, what each receptor contributes, what the trials have found, and what the name does not tell you.

The molecule

Retatrutide is a single peptide, not a mixture. It is engineered to bind and activate three receptors: the receptor for GIP (glucose-dependent insulinotropic polypeptide), the GLP-1 receptor and the glucagon receptor. Like semaglutide and tirzepatide, it is modified to be cleared slowly; the phase 1 work found that its pharmacokinetic profile supported once-weekly dosing.

The discovery paper (Coskun et al., Cell Metabolism 2022) reports that in vitro the peptide shows balanced activity at the glucagon receptor and the GLP-1 receptor, and more activity at the GIP receptor. The relative strength at each receptor was a design choice, and it is the reason retatrutide's effects and side effects are not simply those of a GLP-1 drug plus a glucagon drug.

What each receptor does

The mouse experiments in the discovery paper describe the logic: weight loss from reduced calorie intake, driven by the GIP and GLP-1 components, was augmented by glucagon-receptor-mediated increases in energy expenditure. The glucose-raising effect of glucagon is expected to be offset by the insulin-promoting effect of the two incretin receptors, and in the phase 2 diabetes trial HbA1c fell rather than rose at every dose from 4 mg upward.

From laboratory to phase 3, in dates

The phase 3 figures come from company press releases and have not yet been peer-reviewed. The results article breaks them down by dose and threshold.

What the evidence says about risk

The side-effect profile is dominated by gastrointestinal events that rise with dose. In the phase 2 obesity trial they were mostly mild to moderate and partly reduced by a lower starting dose. Heart rate increased in a dose-dependent way, peaking at 24 weeks. In phase 3, discontinuation because of adverse events reached 11.3% at 12 mg in TRIUMPH-1 and 18.2% at 12 mg in TRIUMPH-4, and a skin-sensation side effect, dysesthesia, appeared at rates of 12% to 21% on the higher doses. These are described in detail in the side effects article and the heart rate article.

Nothing is known from published sources about long-term safety beyond about two years, about use in pregnancy, or about thyroid effects, which for the approved drugs of this class are the subject of a boxed warning based on rodent studies.

What "reta" does not tell you

The short name is used indiscriminately for three different things.

• The trial drug: manufactured by Lilly to pharmaceutical standards, with a known concentration, administered under a protocol. Every number on this site refers to this.
• A future medicine: if approved, retatrutide will have a brand name, a label, defined doses and defined contraindications. As of September 2026 none of these exist. Lilly has said it plans to submit an application to the FDA in the first quarter of 2027; see the FDA approval article.
• A product sold online as "reta": a lyophilized powder in a vial, sold for research use, of a purity and concentration that depend entirely on the seller. It is not regulated as a medicine in any country. The where-to-buy article explains the legal status and what a certificate of analysis can and cannot show.

When someone reports a result or a side effect from "reta" on a forum, it is almost always the third thing. When a study reports a result, it is the first. Keeping the two apart is the single most useful habit for reading about this compound.

Frequently confused terms

• GLP-3. There is no hormone called GLP-3. The term is a marketing shorthand for retatrutide, playing on GLP-1 and the fact that it hits three receptors. See GLP-3 peptide.
• Triple agonist, triagonist, tri-agonist. All describe the same property of retatrutide: agonism at three receptors.
• LY3437943. Lilly's development code, used in the phase 1 papers and in every ClinicalTrials.gov record.
• Retatrutide versus tirzepatide. Tirzepatide is the dual GIP and GLP-1 agonist sold as Mounjaro and Zepbound. Retatrutide adds glucagon. The comparison article sets the numbers side by side.

Where the science is heading

The phase 3 program registered on ClinicalTrials.gov covers obesity with and without type 2 diabetes, obesity with cardiovascular disease, knee osteoarthritis, obstructive sleep apnea (as a substudy of TRIUMPH-1 and TRIUMPH-2), and a 10,000-participant cardiovascular and kidney outcomes trial (TRIUMPH-Outcomes, NCT06383390) with estimated primary completion in February 2029. The results of that last trial will determine whether the heart rate increase seen in phase 2 has any cost in cardiovascular events, and they will not be available for years.