PwPepwise

Retatrutide

Weight Loss

a.k.a. LY3437943

GLP-1 / GIP / glucagon triple agonist

Retatrutide is an injectable peptide drug that simultaneously activates three hormone receptors — GIP, GLP-1, and glucagon receptors.

§Dosing at a glance

4 protocols · from the research
What it's forDoseHow oftenHowFor how long
Obesity (non-diabetic population)4 mgOnce weeklySubcutaneousInjected just under the skin, into the fat layer.48 wks
Type 2 Diabetes0.5 mgOnce weeklySubcutaneousInjected just under the skin, into the fat layer.36 wks
fatty liver/Liver Fat1 mgOnce weeklySubcutaneousInjected just under the skin, into the fat layer.48 wks
Body Composition (T2D substudy)0.5 mgOnce weeklySubcutaneousInjected just under the skin, into the fat layer.36 wks

Approximate values pulled from the research — double-check before dosing.

§01Summary

Retatrutide is an injectable peptide drug that simultaneously activates three hormone receptors — GIP, GLP-1, and glucagon receptors — earning it the designation of a "triple agonist." This triple action sets it apart from older weight-loss medications that target only one or two of these pathways. By engaging all three systems at once, retatrutide reduces appetite, slows digestion, and increases the body's energy expenditure, producing a combined effect on body weight and blood sugar that exceeds what single or dual agents typically achieve.

In clinical trials, retatrutide reduces body weight by up to 24% over 48 weeks in people with obesity — results that approach what is typically seen only with bariatric surgery1. In people with type 2 diabetes, it reduces HbA1c by up to 2% while also producing substantial weight loss2. Early studies also show it dramatically reduces fat accumulation in the liver, normalizing liver fat content in the majority of participants at the highest dose3. Retatrutide is administered as a once-weekly subcutaneous injection and is currently advancing through a broad Phase 3 clinical program spanning obesity, type 2 diabetes, liver disease, kidney disease, and cardiovascular outcomes4,5,6,7.

This is the layperson summary. Mechanism, dosing, the evidence base, and the published literature are in the sections below — every claim links to its source.

§02In depth

Retatrutide (LY3437943) is a synthetic acylated peptide engineered as a triple agonist at the glucose-dependent insulinotropic polypeptide (GIP) receptor, glucagon-like peptide-1 (GLP-1) receptor, and glucagon receptor (GCGR). This trimodal receptor engagement mechanistically distinguishes it from approved GLP-1 receptor agonists (e.g., semaglutide) and dual GIP/GLP-1 agonists (e.g., tirzepatide), potentially conferring additive or synergistic effects on energy homeostasis1,2.

At the GLP-1 receptor, retatrutide stimulates glucose-dependent insulin secretion, suppresses glucagon release in a glucose-dependent manner, slows gastric emptying, and activates hypothalamic satiety pathways to reduce appetite and food intake1,11. GIP receptor agonism augments insulin secretion synergistically with GLP-1 signaling, and in adipose tissue may influence lipid storage and metabolic flexibility2. Glucagon receptor agonism is the mechanistically novel component: glucagon signaling in the liver promotes fatty acid oxidation and increases hepatic glucose output, and in energy balance contexts appears to increase basal metabolic rate and thermogenesis, contributing to greater energy expenditure than dual agonists alone1,3. This hepatic and thermogenic dimension of glucagon receptor agonism likely explains the substantially greater weight loss and liver fat reduction observed with retatrutide compared to GLP-1/GIP dual agonists2,3.

Retatrutide demonstrates dose-proportional pharmacokinetics with an approximately 6-day half-life, pharmacologically supporting once-weekly subcutaneous dosing9. This half-life is achieved through structural acylation engineering, similar to other long-acting fatty acid-conjugated peptide therapeutics, which extends plasma half-life via albumin binding and reduces renal clearance. The compound reaches meaningful plasma concentrations across a stepwise titration schedule starting as low as 0.5–2 mg weekly9.

Downstream metabolic effects are broad: in addition to glycemic and weight outcomes, retatrutide reduces hepatic steatosis through both weight-loss-mediated and direct glucagon receptor-mediated mechanisms promoting hepatic fat oxidation3, improves insulin sensitivity and lipid metabolism markers3, reduces total fat mass with lean mass preservation comparable to other obesity therapies10, and modulates appetite-regulating constructs including perceived hunger, disinhibition, and dietary restraint11. The dose-dependent heart rate elevation observed in Phase 2 — peaking at week 24 and declining thereafter — reflects glucagon receptor-mediated chronotropic activity and is a recognized pharmacodynamic effect of GCGR agonism being characterized in ongoing Phase 3 cardiovascular programs1,5,6.

§04Evidence & efficacy

Evidence base
199Studies
101Human
8Animal

Retatrutide produces robust, dose-dependent reductions in body weight in both obese non-diabetic individuals and people with type 2 diabetes. In the Phase 2 obesity RCT, participants receiving 12 mg achieved a mean body weight reduction of 24.2% at 48 weeks versus 2.1% for placebo, with 100% of participants in the 8 mg and 12 mg groups achieving ≥5% weight loss and 83% of the 12 mg group achieving ≥15% weight loss1. In the Phase 2 type 2 diabetes RCT, retatrutide 12 mg produced weight loss of 16.94% at 36 weeks versus 3.00% for placebo, significantly exceeding both placebo and the active comparator dulaglutide2. Retatrutide also reduces HbA1c by up to 2.02% at 24 weeks with the 12 mg dose in people with type 2 diabetes, statistically superior to both placebo and dulaglutide2.

Retatrutide produces dose-dependent reductions in liver fat ranging from 42.9% at 1 mg to 82.4% at 12 mg at 24 weeks, with liver fat normalization achieved by 86% of participants at the highest dose versus 0% on placebo3. At the body composition level, it produces dose-dependent reductions in total fat mass of up to 26.1% (8 mg pooled) at 36 weeks in the T2D population, with lean mass preservation proportional to other established obesity treatments10. Higher doses (≥4 mg) also reduce appetite, hunger, and disinhibition in people with type 2 diabetes, with eating behavior changes modestly but significantly correlated with weight loss outcomes11. Phase 3 trials investigating cardiovascular outcomes, kidney disease, osteoarthritis, and chronic low back pain are currently underway5,6,7,8.

§05Safety

Retatrutide's safety profile across Phase 1b and Phase 2 trials is consistent with the incretin-based therapy drug class. The most commonly observed adverse events are gastrointestinal in nature — nausea, vomiting, diarrhea, and constipation — and are generally mild to moderate in severity1,2,9. In the Phase 2 obesity trial, gastrointestinal events were dose-dependent, and using a lower 2 mg starting dose partially mitigated these effects compared to a 4 mg starting dose1. In the Phase 2 diabetes trial, gastrointestinal adverse events were reported in 35% of retatrutide-treated participants, ranging from 13% (0.5 mg) to 50% (8 mg fast escalation group), comparable in frequency to dulaglutide2. Faster dose escalation increased the gastrointestinal tolerability burden2.

A dose-dependent increase in heart rate was observed in the Phase 2 obesity trial, peaking at week 24 and declining thereafter1. This cardiovascular signal is actively being characterized in the ongoing Phase 3 cardiovascular outcomes program5,6. No severe hypoglycemia episodes and no deaths were reported in Phase 2 diabetes studies2,10. Serious adverse events were infrequent across treatment arms in the body composition substudy (ranging from 0% to 9%)10. In the Phase 1b trial, 29 of 72 participants (approximately 40%) discontinued prematurely, reflecting tolerability challenges at higher doses during early escalation protocols9. Eli Lilly is actively studying hypoglycemia counterregulatory response in a dedicated mechanistic trial13, as well as safety in vulnerable populations including those with moderate-to-severe renal impairment14 and established cardiovascular disease5,6.

§06History

Retatrutide, also known by its development code LY3437943, was discovered and developed by Eli Lilly and Company as part of a broader effort to engineer multi-receptor incretin-based peptide therapeutics with superior cardiometabolic efficacy. The compound emerged from the conceptual evolution of the incretin field: following the clinical success of GLP-1 receptor agonists in the 2010s and the subsequent development of dual GIP/GLP-1 agonists (exemplified by tirzepatide), investigators sought to incorporate glucagon receptor agonism as a third mechanistic axis to further amplify weight loss via enhanced energy expenditure and hepatic fat oxidation.

The first published human evidence for retatrutide appeared in October 2022, when a Phase 1b multiple-ascending dose trial in people with type 2 diabetes was published in The Lancet, demonstrating dose-proportional pharmacokinetics, a ~6-day half-life supporting once-weekly dosing, and substantial early weight loss and glycemic improvements9. Two landmark Phase 2 RCTs were published simultaneously in June 2023 in the New England Journal of Medicine — one in obesity1 and one in type 2 diabetes2 — generating immediate high-impact attention, with the obesity trial accumulating over 935 citations within approximately two years1. A Phase 2a MASLD trial published in Nature Medicine in 2024 extended the evidence base to liver disease3.

By 2023–2025, Eli Lilly had initiated a comprehensive Phase 3 program — branded TRIUMPH — spanning obesity, type 2 diabetes, cardiovascular outcomes, kidney disease, MASLD, osteoarthritis, and chronic pain4,5,6,7,8,12,14,20, representing one of the broadest development programs for any single metabolic peptide therapeutic. Regulatory submissions are anticipated as pivotal Phase 3 data mature.

§07References