Retatrutide: The State of Research in 2026

Abstract laboratory-style illustration: three molecular pathways converging on a central receptor, symbolizing retatrutide's triple agonism of GIP, GLP-1 and glucagon.

Retatrutide at a Glance: What the Published Trials Show

Retatrutide (development code LY3437943) is an investigational peptide developed by Eli Lilly and the first triple agonist of the GIP, GLP-1 and glucagon receptors to reach phase 3. In the phase 2 trial published in the New England Journal of Medicine in 2023 (Jastreboff et al.), participants on the highest weekly dose showed a mean weight reduction of 24.2% at 48 weeks, versus 2.1% on placebo — at the time, the largest reduction reported for any anti-obesity drug in development.

The TRIUMPH phase 3 program delivered its first results between December 2025 and July 2026: up to 28.3% mean weight loss at 80 weeks in TRIUMPH-1, and 30.3% at 104 weeks in that trial's extension, according to Eli Lilly's announcements. A substudy published in Nature Medicine additionally reported normalization of liver fat in more than 85% of participants with steatotic liver disease at the highest doses. Retatrutide is not approved by any regulatory agency: Lilly has stated it plans to file with the FDA in the first quarter of 2027. This article reports the state of the literature for scientific information purposes; retatrutide remains a compound strictly reserved for research.

What Is Retatrutide (LY3437943)?

Retatrutide is a synthetic peptide built on the backbone of GIP (glucose-dependent insulinotropic polypeptide) and engineered to activate three receptors at once: the GIP receptor, the GLP-1 (glucagon-like peptide-1) receptor and the glucagon receptor. A fatty-acid moiety attached to the sequence extends its half-life, allowing once-weekly subcutaneous administration in clinical trials. It belongs to the incretin-based lineage that produced semaglutide (a single GLP-1 agonist) and then tirzepatide (a dual GIP/GLP-1 agonist), adding a third pathway of action.

Developed by Eli Lilly, the compound moved through preclinical and phase 1 work in the early 2020s before two phase 2 trials were published simultaneously in June 2023 — one in obesity (NEJM), the other in type 2 diabetes (The Lancet). The phase 3 program, named TRIUMPH, was launched the same year and spans obesity, type 2 diabetes, knee osteoarthritis, obstructive sleep apnea and people with established cardiovascular disease. To date, retatrutide remains an experimental molecule with no marketing authorization anywhere in the world.

Mechanism: Why a GIP/GLP-1/Glucagon Triple Agonist?

Each of the three targeted pathways plays a distinct role in energy regulation. GLP-1 receptor activation is associated in the literature with reduced appetite, slower gastric emptying and glucose-dependent stimulation of insulin secretion. GIP, long considered a minor player, appears to potentiate these metabolic effects and may improve the gastrointestinal tolerability of GLP-1 agonism, as tirzepatide data suggest.

What sets retatrutide apart is the addition of glucagon receptor agonism. Unlike the first two pathways, which act mainly on food intake, glucagon increases energy expenditure and stimulates hepatic lipid oxidation. The hypothesis researchers are testing is that combining reduced intake (GLP-1, GIP) with increased expenditure (glucagon) yields greater weight loss than single or dual agonists — a hypothesis the phase 2 and phase 3 results appear to support, although no randomized head-to-head comparison with tirzepatide has been published. The peptide's pharmacological profile is deliberately imbalanced, with relatively greater activity at the GIP receptor than at the other two.

Phase 2: The Results Published in NEJM and The Lancet (2023)

The phase 2 obesity trial (Jastreboff et al., NEJM, June 2023; NCT04881760) randomized 338 adults with obesity or overweight, without diabetes, to placebo or weekly retatrutide at 1, 4, 8 or 12 mg for 48 weeks. Mean weight reductions at 48 weeks were 8.7% (1 mg), 17.1% (4 mg), 22.8% (8 mg) and 24.2% (12 mg), versus 2.1% on placebo. At the 12 mg dose, 83% of participants lost at least 15% of their baseline weight. Notably, the weight curves had not plateaued by the end of the trial, suggesting larger reductions over longer durations.

Published in parallel in The Lancet, the phase 2 trial in 281 people with type 2 diabetes (Rosenstock et al., 2023) reported, at 36 weeks, reductions in glycated hemoglobin of up to roughly 2 percentage points and weight loss approaching 17% at the highest dose — greater than the active comparator dulaglutide 1.5 mg. Together, these two trials established the dose range and the stepwise escalation scheme later carried into phase 3.

Phase 3 TRIUMPH: The Results Announced in 2025-2026

The first phase 3 trial to read out was TRIUMPH-4 (NCT05931367), conducted in 445 adults with obesity and knee osteoarthritis: according to Eli Lilly's December 11, 2025 announcement, mean weight loss at 68 weeks reached 28.7% at 12 mg (versus 2.1% on placebo), with knee pain improvement (WOMAC score) of roughly 74-76% versus 40% on placebo. On May 21, 2026, Lilly announced results from the pivotal TRIUMPH-1 trial (NCT05929066), which randomized more than 2,300 adults with obesity or overweight without diabetes: mean weight loss of 19.0% (4 mg), 25.9% (9 mg) and 28.3% (12 mg) at 80 weeks. In the prespecified 104-week extension, the 12 mg dose was associated with a mean loss of 30.3%, and 45.3% of participants at that dose lost at least 30% of their baseline weight.

On July 23, 2026, the company reported results from two further trials. TRIUMPH-2, in 1,152 adults with type 2 diabetes, reported mean weight loss of up to 20.8% at 80 weeks and HbA1c reductions of up to 1.6 points. TRIUMPH-3, in 1,949 adults with severe obesity and established cardiovascular disease, reported up to 22.6% mean weight loss; exploratory analyses of cardiovascular events are described, but the trial was not powered to conclude on those endpoints and researchers point to dedicated outcome studies. These figures come from press releases: full peer-reviewed publications with complete datasets are expected at conferences and in journals from late 2026 onward.

Liver Steatosis: Notable Data in Nature Medicine

A substudy of the phase 2 obesity trial, published in Nature Medicine in June 2024 (Sanyal et al.), evaluated 98 participants with metabolic dysfunction-associated steatotic liver disease (MASLD) and at least 10% liver fat measured by MRI. At 24 weeks, the mean relative reduction in liver fat reached 81.4% (8 mg) and 82.4% (12 mg), versus near-stability on placebo; at 48 weeks it reached 86.0% at the highest dose.

More than 85% of participants on the two highest doses had normalized liver fat — below 5% — at 48 weeks. The authors attribute part of this effect to the glucagon component of the triple agonism, which stimulates hepatic lipid oxidation, and report no hepatotoxicity signal over the study period. These results still come from a modestly sized phase 2a study based on imaging rather than histology: they do not allow conclusions about fibrosis or steatohepatitis (MASH), which are the subject of separate investigations.

Adverse Events Reported in the Trials

The reported tolerability profile is broadly consistent with the incretin-based class, dominated by dose-dependent gastrointestinal effects. In TRIUMPH-1, the most frequent events were nausea (28.6% to 42.4% depending on dose), diarrhea (25.2% to 34.1%) and constipation (23.8% to 26.1%), mostly mild to moderate and occurring during dose escalation. Discontinuations due to adverse events ranged from 4.1% to 11.3% in TRIUMPH-1 and up to 18.2% at the highest dose in TRIUMPH-4, versus about 4% on placebo.

The NEJM phase 2 trial had additionally reported a dose-dependent increase in heart rate, peaking around week 24 and declining thereafter, as well as cases of cutaneous hyperesthesia or dysesthesia in a minority of participants — signals the full phase 3 publications will need to clarify. As with other molecules in the class, long-term data beyond two years, rare events, and effects in populations excluded from the trials (pregnancy, specific medical histories, very young or very old subjects) remain uncharacterized at this stage.

What Research Does Not Yet Know

Several major questions remain open. The durability of weight loss after stopping the compound has not been reported for retatrutide, whereas withdrawal studies of other incretin-based agents show substantial weight regain. The composition of the weight lost — the proportion of lean versus fat mass — is documented only in limited substudies. No randomized trial directly comparing retatrutide with tirzepatide or semaglutide has been published, which rules out formal efficacy comparisons despite apparently superior numbers.

On the cardiovascular side, the net effect of triple agonism — combining expected metabolic benefits with a transient glucagon-related rise in heart rate — will need to be settled by long-term event data, as the available TRIUMPH-3 analyses are exploratory. Finally, effects on liver histology (fibrosis, MASH), renal outcomes and hard morbidity-mortality endpoints are not yet established. Caution dictates treating figures drawn from press releases as provisional until the peer-reviewed publications are available.

Regulatory Status and Research Framing

As of August 2026, retatrutide is not approved by any health authority — not the US FDA, not the European EMA, not France's ANSM. According to Eli Lilly's July 23, 2026 announcement, the company plans to submit a licensing application to the FDA in the first quarter of 2027 on the basis of the TRIUMPH program; European timelines have not been specified. Any eventual pharmaceutical availability would therefore begin, at the earliest, in 2027-2028, and only in the indications validated by regulators.

One essential point follows: any retatrutide circulating today outside clinical trials is an unapproved product, not controlled by any pharmaceutical authority, and nothing allows the results obtained with Lilly's pharmaceutical-grade material to be extrapolated to such products. The peptides offered on this site fall exclusively under the Research Use Only category: they are intended for in vitro research and laboratory work by professionals, and are neither medicines, nor supplements, nor products intended for human or veterinary administration. This article does not constitute medical advice in any way; people affected by obesity or diabetes should consult a healthcare professional.

Sources

Keep reading

EncyclopediaReta-R3