Peptide fact-sheet · English

Retatrutide

Triple GIP, GLP-1, and glucagon receptor agonist

Triple GIP/GLP-1/glucagon receptor agonist in phase 3; mean weight loss 24.2% at 48 weeks in phase 2.

Translated and adapted from the canonical Portuguese version: /peptideos/retatrutida.

Quick answer

Retatrutide (LY3437943) is a synthetic peptide triple agonist of the GIP, GLP-1, and glucagon receptors developed by Eli Lilly. The phase 2 obesity trial (Jastreboff 2023, NEJM, PMID 37366315, n=338, 48 weeks) reported mean weight loss of -24.2% at the 12 mg dose versus -2.1% with placebo — the largest magnitude observed in a published trial of a single peptide molecule for obesity at the time. The phase 2 type 2 diabetes trial (Rosenstock 2023, Lancet, PMID 37364590, n=281, 36 weeks) reported HbA1c reductions up to -2.02% at 12 mg. A phase 2a MASLD substudy (Sanyal 2024, Nature Medicine, PMID 38858523, n=98) reported liver fat reductions of 87-93% at 8 and 12 mg. The phase 3 TRIUMPH program is ongoing with four principal trials (TRIUMPH-1 through TRIUMPH-4) enrolling more than 5,800 participants across obesity, type 2 diabetes, established cardiovascular disease, and knee osteoarthritis. As of May 2026, retatrutide has no regulatory approval in any country. The most distinctive safety signal is a dose-dependent increase in heart rate of 6-9 bpm at the 8 and 12 mg doses, attributed to the glucagon receptor component and under prospective evaluation in TRIUMPH-3.

What it is

Retatrutide is a synthetic peptide developed by Eli Lilly under code LY3437943. It is the first peptide molecule to reach phase 3 development as a balanced agonist at three receptors involved in energy metabolism: GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1), and glucagon. The structural backbone is acylated, prolonging plasma half-life to approximately 6 days and supporting once-weekly subcutaneous dosing.

The therapeutic hypothesis is mechanistic. GLP-1 receptor agonism produces weight loss through pancreatic, gastric, and hypothalamic actions. Adding GIP receptor agonism (as in tirzepatide) amplifies the metabolic effect with a tolerability profile comparable to GLP-1 monotherapy. Adding glucagon receptor agonism — the strategy retatrutide pursues uniquely at this stage of development — introduces hepatic actions (fatty acid oxidation, increased basal metabolic rate, thermogenesis in brown adipose tissue) that preclinical models predicted would yield weight loss exceeding incretin-only agonism. The phase 2 obesity result is consistent with that prediction.

As of May 2026, retatrutide has no regulatory approval in any jurisdiction and is not available outside of registered clinical trials. The molecule has no INN registered with the WHO.

Mechanism of action

Retatrutide engages three class B G-protein-coupled receptors simultaneously:

GLP-1 receptor — glucose-dependent insulin secretion, glucagon suppression in the postprandial state, delayed gastric emptying, central appetite regulation through hypothalamic and brainstem signaling.
GIP receptor — amplification of glucose-dependent insulin response, modulation of adipose tissue insulin sensitivity, and a gastrointestinal-event profile that is typically more favorable than selective GLP-1 receptor agonism at comparable efficacy.
Glucagon receptor — hepatic fatty acid oxidation, increased basal energy expenditure, induction of thermogenesis in brown adipose tissue, and modulation of hepatic gluconeogenesis. In monotherapy, glucagon receptor agonism raises plasma glucose; in the context of concurrent GIP and GLP-1 agonism, the insulinotropic incretin effect offsets the hyperglycemic potential.

The integration is the central design feature. Retatrutide is not three drugs in one; it is a single molecule with balanced affinity for the three receptors, engineered so that the glucagon component contributes weight loss and hepatic improvement without compromising glycemic control. The phase 2 type 2 diabetes trial supports this: HbA1c reductions were comparable in magnitude to those reported for tirzepatide and semaglutide despite the added glucagon agonism.

The molecule-specific signal of dose-dependent heart-rate elevation in phase 2 (6-9 bpm at 8 and 12 mg) is attributed to the glucagon receptor component and is the principal mechanistic question that the phase 3 TRIUMPH program is designed to clarify, particularly in TRIUMPH-3 (participants with established cardiovascular disease).

What the trials show

Phase 2 obesity — Jastreboff 2023 (NEJM, PMID 37366315, NCT04881760). Multicenter randomized double-blind placebo-controlled dose-escalation trial, 48 weeks, n=338 adults with BMI ≥30 or ≥27 with at least one weight-related comorbidity, without type 2 diabetes. Retatrutide 1, 4, 8, or 12 mg weekly vs placebo. Least-squares mean weight changes at 48 weeks:

-2.1% placebo
-8.7% retatrutide 1 mg
-17.1% retatrutide 4 mg (combined)
-22.8% retatrutide 8 mg (combined)
-24.2% retatrutide 12 mg

≥15% weight loss occurred in 60% (4 mg), 75% (8 mg), and 83% (12 mg) of participants versus 2% with placebo. Gastrointestinal events were the most common adverse events, dose-related, mostly mild to moderate, and partially mitigated by starting at 2 mg rather than 4 mg.

Phase 2 type 2 diabetes — Rosenstock 2023 (Lancet, PMID 37364590, NCT04867785). Multicenter randomized double-blind placebo- and dulaglutide-controlled trial, 36 weeks, n=281 adults with type 2 diabetes. HbA1c reductions: up to -2.02% at retatrutide 12 mg vs -0.01% placebo and -1.41% dulaglutide 1.5 mg. Weight loss up to -16.94% at 12 mg.

Phase 2a MASLD substudy — Sanyal 2024 (Nature Medicine, PMID 38858523). Substudy of NCT04881760 in 98 participants with metabolic dysfunction-associated steatotic liver disease characterized by MRI-PDFF. Liver fat content reductions at 48 weeks: 10% placebo, 87.1% retatrutide 8 mg, 93.2% retatrutide 12 mg. The most robust hepatic-fat reduction signal published for a single peptide molecule to date. Histologic confirmation in biopsy is the next step required for a MASH regulatory indication.

Phase 3 TRIUMPH program — Jastreboff 2025 (Diabetes Obesity & Metabolism, PMID 41090431). Methods publication describing the four registrational trials:

TRIUMPH-1 (NCT05929066) — 80 weeks, obesity or overweight with embedded sub-protocols for obstructive sleep apnea and osteoarthritis.
TRIUMPH-2 (NCT05929079) — 80 weeks, obesity or overweight with type 2 diabetes.
TRIUMPH-3 (NCT05882045) — 80 weeks, obesity or overweight with established cardiovascular disease; cardiovascular outcomes as a registrational endpoint.
TRIUMPH-4 (NCT05931380) — 68 weeks, obesity or overweight with knee osteoarthritis. Eli Lilly reported in 2026 that the trial met its primary and key secondary endpoints, with weight loss reaching up to -28.7% over 68 weeks alongside improvements in pain and function.

Aggregate enrollment exceeds 5,800 participants. Full peer-reviewed publications of TRIUMPH-1 through TRIUMPH-3 are anticipated in 2026-2027.

Pharmacokinetics

Subcutaneous half-life: approximately 6 days, supporting once-weekly dosing.
Investigational dosing schedule (phase 2 and 3): 1, 2, 4, 8, 9, and 12 mg weekly. Stepwise escalation over 4-week intervals from a 2 mg starting dose is used in current phase 3 protocols to mitigate gastrointestinal events.
Elimination: characteristic of acylated peptides — proteolytic degradation followed by renal and fecal excretion of fragments.

Safety and adverse events

The phase 2 safety database covers approximately 700 participants exposed to retatrutide across the three principal trials, with additional exposure accruing in the phase 3 TRIUMPH program.

Gastrointestinal events — most common, dose-dependent, predominantly during titration, generally mild to moderate. Frequencies in the obesity phase 2 trial: nausea 28% (1 mg) to 56% (12 mg) vs 13% placebo; diarrhea 24-30% vs 19%; vomiting up to 35% at higher doses vs 8% placebo. Gastrointestinal-related discontinuation was higher at 8 and 12 mg but consistent with the class.
Dose-dependent heart-rate elevation — mean increase of 6-9 bpm at 8 and 12 mg, attributed to glucagon receptor agonism. Molecule-specific signal. Clinical significance during prolonged use is the focus of TRIUMPH-3.
Thyroid C-cell tumor signal — class precaution carried over from preclinical rodent data on GLP-1 receptor agonists. No characterized human event in published phase 2.
Acute pancreatitis — isolated phase 2 events, magnitude consistent with the class; no statistically meaningful excess over placebo in phase 2.
Hypoglycemia — rare in monotherapy. Expected in combination with sulfonylureas or insulin, by class extrapolation.

Safety during continuous use beyond 80 weeks is not characterized in peer-reviewed literature. Any claim about retatrutide safety beyond 12-18 months as of mid-2026 is an extrapolation.

Regulatory status (international)

FDA: no approval. Phase 3 TRIUMPH ongoing; submissions anticipated 2027-2028.
EMA: no approval.
INN (WHO): not assigned.
WADA: not listed by name in the 2026 Prohibited List. The GLP-1 class is not classified as prohibited; athletes should confirm with their national anti-doping organization.

Material sold internationally under the label "retatrutide" through "research peptides" catalogs since 2023 is not characterized by audited HPLC or peptide-mapping analysis and represents irregular supply. Distinguishing authentic retatrutide from tirzepatide (closely related in sequence) or from peptide-fragment mixtures without independent analytical characterization is not a trivial task.

What we still don't know

Peer-reviewed phase 3 outcomes from TRIUMPH-1, TRIUMPH-2, and TRIUMPH-3.
Head-to-head randomized data versus tirzepatide and semaglutide.
Long-term clinical significance of the dose-dependent heart-rate elevation, particularly in adults with established cardiovascular disease.
Histologic confirmation in biopsy for a MASH regulatory indication.
Safety and pharmacovigilance signal beyond 80 weeks.
Exact regulatory timelines: FDA/EMA submissions anticipated 2027-2028, but contingent on completion and integrity of the TRIUMPH dataset.

Why it matters

Retatrutide represents the current leading edge of incretin-based pharmacotherapy for obesity and cardiometabolic disease. The phase 2 weight-loss magnitude (-24.2% at 12 mg over 48 weeks) exceeded the historical ceiling for any published trial of a single peptide molecule for obesity and reframed expectations for the class. Whether that magnitude sustains in the larger and longer phase 3 TRIUMPH program — and whether the heart-rate signal proves clinically tolerable, particularly in TRIUMPH-3 — are the questions that will determine the molecule's clinical position. Until phase 3 peer-reviewed results and regulatory decisions are in, retatrutide remains investigational. Any clinical use outside a registered trial is not supported by current evidence or regulatory frameworks.