Retatrutide: what the research literature shows

What it is

Origin and names

Retatrutide comes from Eli Lilly and Company, whose scientists described its discovery in 2022 under the code LY3437943.¹ The Global Substance Registration System, a United States government registry, records “retatrutide” as the international nonproprietary name and LY3437943 as a code.⁵

It belongs to the family of incretin receptor agonists, alongside semaglutide, which activates the GLP-1 receptor only, and tirzepatide, which activates the GIP and GLP-1 receptors; neither targets the glucagon receptor.¹²

Sequence and modifications

The registry describes a single chain of 39 amino acids that ends in an amide group.⁵ Three positions hold residues that are not among the twenty standard amino acids: α-aminoisobutyric acid (Aib) at positions 2 and 20, and α-methyl-leucine at position 13.⁵ The lysine at position 17 carries a side chain built from a short ethylene glycol spacer, a γ-glutamic acid and a 20-carbon fatty diacid.⁵ A structural study describes the sequence as developed from the backbone of the hormone GIP.¹²

PubChem computes the formula C221H342N46O68 and a molecular weight of about 4,731 g/mol for this structure.¹³ The database files the structure under more than one entry, and the fully specified one is titled as the sodium salt.¹³

Review A 2023 review describes attaching a fatty acid as an established way to prolong the action of peptides and proteins, first applied to insulin;¹⁴ the guide to half-life extension strategies explains how.

How it is studied

• Cell and structural assays. The rise in cyclic AMP, a messenger molecule, is measured in cells that carry a given receptor; cryo-electron microscopy shows how the peptide sits in each receptor.¹²
• Isolated tissue. Preparations of atrial heart muscle are tested for changes in force and beating rate.¹⁵¹⁶
• Animal models. Obese mice and hamsters are followed for body weight, food intake, energy expenditure and liver fat.¹¹⁷
• Randomized clinical trials. Double-blind, placebo-controlled trials record change in body weight and in HbA1c alongside adverse events.⁷¹⁰

The guide to levels of evidence in peptide research explains what each tier can show.

What the literature reports

Receptor pharmacology and structure

Cell study In the discovery report, LY3437943 activated all three receptors, with balanced activity at the glucagon and GLP-1 receptors and more activity at the GIP receptor.¹ A later cryo-electron microscopy study solved structures of retatrutide bound to each of the three receptors; in them the peptide forms one continuous helix whose first residues reach into the receptor core.¹² The authors attribute the triple activity to contacts that all three receptors share, plus contacts specific to each one.¹²

Cell study In human atrial muscle taken during heart surgery, retatrutide increased the force of contraction, and blockers of each of the three receptors reduced that effect.¹⁵ In a mouse study from the same laboratory it raised the beating rate of right atrial tissue, an effect that a glucagon receptor blocker stopped, and did not raise the force of contraction in left atrial tissue.¹⁶

Animal models

Animal study In obese mice, the discovery report found lower body weight and what its authors call improved glycemic control with LY3437943.¹ The authors attributed part of the weight loss to higher energy expenditure driven by the glucagon receptor, added to lower calorie intake driven by the GIP and GLP-1 receptors.¹

Animal study A 2026 study from Physiogenex and Janvier Labs tested retatrutide in diet-induced obese mouse and hamster models of steatohepatitis, a form of fatty liver disease.¹⁷ In mice, body weight fell by 31%, a significant difference from controls; both fat and lean mass were lost, and energy expenditure did not change significantly.¹⁷ In hamsters, liver triglyceride content fell by half while the histology score did not fall.¹⁷ The two mouse reports therefore differ on energy expenditure.¹¹⁷

Phase 1 trials

Human study The discovery report included a phase 1 single-ascending-dose study, and its authors described the safety and tolerability profile as similar to that of other incretins.¹ A 12-week phase 1b trial randomized 72 adults with type 2 diabetes to ascending dose cohorts of LY3437943, placebo or the comparator drug dulaglutide, with safety and tolerability as the primary outcome.⁶ Adverse events were reported by 63% of participants on LY3437943, 60% on dulaglutide and 54% on placebo, most often gastrointestinal, and 29 participants left the study early.⁶ The measured half-life was about six days.⁶ Against placebo, plasma glucose and HbA1c fell significantly in the three highest dose cohorts, and the fall in body weight appeared dose-dependent.⁶

Phase 2 trials

Human study The phase 2 obesity trial randomized 338 adults with obesity or overweight to retatrutide dose groups or placebo for 48 weeks.⁷ At 48 weeks, mean body weight had fallen by 8.7% in the lowest dose group and 24.2% in the highest, against 2.1% with placebo.⁷

Human study The phase 2 diabetes trial randomized 281 adults with type 2 diabetes to retatrutide dose groups, placebo or dulaglutide.¹⁰ At 24 weeks, mean HbA1c had fallen by 2.02 percentage points in the highest dose group, against 0.01 with placebo and 1.41 with dulaglutide.¹⁰ At 36 weeks, mean body weight had fallen by 16.94% in that group, against 3.00% with placebo.¹⁰

Human study Two substudies added tissue measures. Among 98 obesity-trial participants with fatty liver disease, the mean relative fall in liver fat at 24 weeks ranged from 42.9% to 82.4% across dose groups, against a 0.3% rise with placebo.¹⁸ In the diabetes trial, total fat mass at 36 weeks fell by 4.9% to 26.1% across dose groups, against 4.5% with placebo; the lowest dose group did not differ significantly from placebo.¹⁹

Phase 3 trials

Human study The developer’s TRIUMPH program comprises four phase 3 trials in more than 5,800 participants; two nest sleep apnea or knee osteoarthritis protocols inside a weight management trial.²⁰

Human study TRIUMPH-1 randomized 2,339 adults with obesity and without diabetes to three retatrutide dose groups or placebo for 80 weeks.² Mean body weight fell by 17.6%, 23.7% and 25.0% in the retatrutide groups and by 3.9% with placebo.² In nested subgroups with knee osteoarthritis or obstructive sleep apnea, pain scores and breathing events per hour fell further in the two higher dose groups than with placebo.²

Human study TRIUMPH-2 randomized 1,152 adults with obesity and type 2 diabetes at 92 sites in eight countries.⁸ At 80 weeks, mean body weight had fallen by 11.9%, 16.8% and 18.8% in the retatrutide groups and by 5.1% with placebo.⁸

Human study TRANSCEND-T2D-1 randomized 537 adults with type 2 diabetes not adequately controlled by diet and exercise alone, for 40 weeks.⁹ Mean HbA1c fell by 1.69, 1.86 and 1.94 percentage points in the retatrutide groups and by 0.81 with placebo.⁹

Adverse events in the trials

Human study In the phase 2 obesity trial, gastrointestinal events were the most common adverse events, were dose-related and were mostly mild to moderate.⁷ Heart rate rose in a dose-dependent way, peaked at 24 weeks and declined afterwards.⁷ In the phase 2 diabetes trial, mild-to-moderate gastrointestinal events were reported by 35% of participants on retatrutide, 13% on placebo and 35% on dulaglutide.¹⁰ Gastrointestinal events were also the most common adverse events in TRIUMPH-1.²

Human study In TRIUMPH-2, diarrhea occurred in 27% to 34% of retatrutide participants against 13% on placebo, and nausea in 14% to 28% against 8%.⁸ Low blood pressure was reported in up to 6% of retatrutide participants and dysesthesia, an abnormal skin sensation, in up to 7%, each at or below 1% on placebo.⁸ Permanent discontinuation because of adverse events or death was 4%, 12% and 8% across the ascending retatrutide dose groups and 5% on placebo.⁸ Seven participants died during the trial, six in retatrutide groups and one on placebo, and investigators judged every death unrelated to the study treatment.⁸

Human study In TRANSCEND-T2D-1, 2% to 5% of retatrutide participants stopped treatment because of adverse events and none on placebo did.⁹ Two deaths occurred in one retatrutide group and were reported as unrelated to the study drug.⁹ No severe hypoglycemia (low blood glucose) was reported in that trial or in the phase 2 diabetes trial.⁹¹⁰

Human evidence

Human study Retatrutide has been tested in randomized, placebo-controlled trials in phase 1, phase 2 and phase 3, and the largest published trial enrolled 2,339 people.²⁶⁷ The longest published follow-up is 80 weeks.²⁸

A phase 3 trial that counts major cardiovascular and kidney events has an estimated primary completion date in 2029.²¹ A phase 2b mechanistic study of kidney function has published its design and baseline data; no results paper is cited here.²² Registered phase 3 comparisons with tirzepatide and with semaglutide were still active on 8 October 2026.¹¹

Regulatory status

Each statement below was checked against the authority’s own page on 8 October 2026.

• Canada. A search of Health Canada’s Drug Product Database for retatrutide as an active ingredient returned no product.³ In a public advisory dated 9 April 2026, Health Canada named retatrutide among examples of unauthorized peptide drugs it had seized.²³ It said that such products have not been assessed for safety, efficacy and quality, and that a “For Research Use Only” label does not make them legal or exempt them from regulatory requirements.²³
• United States. A search of the FDA’s approved-drug data for retatrutide as an active ingredient returned no match.⁴ The FDA states that retatrutide is not a component of any FDA-approved drug, has not been found safe and effective for any condition, and cannot be used in compounding.²⁴ The same page says the agency has warned companies that illegally sold unapproved drugs containing retatrutide or related compounds, falsely labelled “for research purposes” or “not for human consumption”, directly to consumers.²⁴
• Registered trials. ClinicalTrials.gov listed 33 studies with retatrutide as an intervention: 14 in phase 1, four in phase 2, 14 in phase 3 and one expanded-access record.¹¹ Of the phase 3 studies, five were completed and nine were active or recruiting.¹¹

Horizon Peptides supplies retatrutide for laboratory research use only.

In the laboratory

Horizon supplies retatrutide as a lyophilized powder in sealed vials; see the retatrutide vial. The catalogue also lists a pre-mixed solution in a pen.

A 2026 chemistry paper states that retatrutide is made mainly by solid-phase peptide synthesis and describes a liquid-phase alternative.²⁵ One validated method detects intact retatrutide in human plasma by liquid chromatography with high-resolution mass spectrometry, reading the triply charged ion.²⁶ Batch documents are published on the Lab Results page when they exist.

No stability study of retatrutide is cited on this page; general rules are in the peptide storage guide.