Retatrutide: what the trials show

What retatrutide is

Retatrutide is the international nonproprietary name for LY3437943, a synthetic 39-residue peptide developed by Eli Lilly. It is built on a glucose-dependent insulinotropic polypeptide (GIP) backbone with substitutions chosen so that a single molecule engages three receptors: the GLP-1 receptor (GLP-1R), the GIP receptor (GIPR) and the glucagon receptor (GCGR). Its balance across the three is deliberately uneven. In the published receptor-pharmacology work it is described as most potent at GIPR, with lower relative activity at GLP-1R and GCGR than the native ligands have at their own receptors.

A C20 fatty-diacid arm is attached to a lysine side chain through a linker. That lipid is not decorative: it binds serum albumin, which protects the peptide from renal clearance and enzymatic degradation and gives it a functional half-life measured in days rather than minutes. Once-weekly kinetics were a design target from the outset, in the same way they were for semaglutide and tirzepatide, and they are why the trial programme was built around a weekly schedule. The kinetics are a property of the molecule; the schedules built on them are protocol and live at /protocols/retatrutide-20mg/, not here.

The molecule is supplied as a lyophilised, acetate-buffered powder. That formulation detail matters later, in the section on reconstitution.

How it differs from tirzepatide and semaglutide

The three are usually presented as a ladder. Semaglutide is a single agonist at GLP-1R. Tirzepatide is a dual agonist at GIPR and GLP-1R, also 39 residues, also carrying a C20 fatty-diacid arm. Retatrutide adds a third arm, glucagon-receptor activity, to what is otherwise a closely related design. Structurally, tirzepatide and retatrutide are siblings: same length, same class of lipidation, same GIP-derived scaffold. The difference is which receptors the substituted residues are tuned to engage.

Mechanistically the receptors do different things. GLP-1R activation is associated with insulin secretion, slowed gastric emptying and central appetite signalling. GIPR activation is studied for its own effects on insulin secretion and adipose tissue, and for how it modifies the tolerability of GLP-1R signalling when the two are combined. GCGR activation is the odd one out, because glucagon raises hepatic glucose output; its inclusion is a bet that its effects on energy expenditure and hepatic lipid handling outweigh that, provided the incretin arms hold glucose in check. The next section on the glucagon arm covers that hypothesis in more detail.

For laboratory work the practical consequence is that the three make a natural comparison set. Receptor-engagement assays, cAMP and beta-arrestin recruitment, and metabolic phenotype models run across a mono-, dual- and triple-agonist series can separate what each additional receptor contributes. That is the context in which our library entries for semaglutide and tirzepatide describe them, and it is the main reason all three are stocked as reference standards.

The trial programme, by phase and endpoint

Retatrutide has been through phase 1 pharmacokinetic and safety studies and phase 2 trials in obesity, in type 2 diabetes, and in participants with obesity and metabolic dysfunction-associated steatotic liver disease (MASLD). The phase 2 obesity and type 2 diabetes trials were published in peer-reviewed journals in 2023, with the liver substudy reported alongside them. A phase 3 programme, TRIUMPH, was then registered across obesity, obesity with knee osteoarthritis, obesity with cardiovascular or kidney disease, and type 2 diabetes, with the first phase 3 results reported from 2025 onward.

The endpoints are the useful thing to understand, because they define what the literature can and cannot tell you. The obesity trials measured percentage change in body weight from baseline, against placebo. The diabetes trials measured change in HbA1c, with weight as a secondary endpoint, against placebo and an active comparator. The MASLD substudy measured liver fat fraction by MRI-PDFF, with normalisation of liver fat as a secondary outcome. Across those studies the reported effects were dose-dependent, and the published discussion frames the weight and liver-fat results as larger than those seen with earlier single- and dual-agonist trials of similar duration, while noting that cross-trial comparison is not a controlled test.

The specific figures are in the papers and are easy to find. They are deliberately not repeated here, because a percentage lifted from a clinical trial and placed on a supplier's product page reads as a promise about a product, and it is not one. Retatrutide is an investigational compound. As of the date on this page it has no marketing authorisation from Health Canada, the FDA, the EMA or any other regulator, and the material we supply is a research reference standard, not the clinical product those trials used.

The glucagon-receptor arm, and what it is hypothesised to add

Glucagon is the counter-regulatory hormone to insulin. In the liver it drives glycogen breakdown and gluconeogenesis, which is why raising it looks like a strange choice in a metabolic-disease programme. The rationale is that glucagon does more than raise glucose. In preclinical and early human work, GCGR agonism is associated with increased energy expenditure, increased hepatic fatty-acid oxidation and reduced hepatic lipid content, and with a satiety effect that runs separately from the GLP-1 pathway.

The hypothesis behind a triple agonist is therefore a division of labour. The GLP-1R and GIPR arms are expected to reduce intake and improve glucose handling; the GCGR arm is expected to raise the energy-expenditure side of the balance and act directly on liver fat, with the incretin arms offsetting the glucose-raising effect glucagon would otherwise have. Whether that offset holds is an empirical question, and the type 2 diabetes phase 2 trial was designed partly to test it.

Two features of the literature follow from the glucagon arm. The first is the interest in MASLD, where hepatic fat is the endpoint that GCGR activity is most directly hypothesised to act on. The second is the heart-rate observation discussed in the next section, since both glucagon and GLP-1 have chronotropic effects. Neither of these is a finding this page can state as a benefit; they are the questions the programme is set up to answer.

Safety signals the literature discusses

The published trials report adverse events in the same neutral form as the efficacy endpoints, and they are worth reading the same way: as what was observed in a supervised clinical population under a fixed protocol, not as guidance for anyone. Three themes recur across the papers and the accompanying editorials.

Gastrointestinal tolerability is the dominant one. Nausea, diarrhoea, vomiting and constipation were the most frequently reported adverse events, as they are across the GLP-1 class, and they were dose-related and concentrated during the escalation period. The trials used stepwise escalation partly to manage this, and discontinuation because of adverse events was mostly gastrointestinal.

Heart rate is the second. A mean increase in resting heart rate was observed, larger at higher dose levels and tending to moderate with continued treatment. It is consistent with what is seen with GLP-1R agonists and is compounded in principle by the glucagon arm. Related to that, the trials monitored for cardiac arrhythmia; the papers report the incidence of arrhythmia-related adverse events and the phase 3 programme includes a dedicated cardiovascular-outcomes component. Other observations discussed include skin paraesthesia at higher doses and the changes in glucose and lipid parameters expected of the class. A physician is the person to interpret any of this for an individual, and this page is not one.

Sizes, storage and reconstitution

We stock retatrutide in two vial sizes, 10 mg and 20 mg, as lyophilised powder in sealed vials. The choice between them is a question of how much material a study needs and price per milligram, which is printed on the product page; a single larger vial is the cheaper way to buy a given mass, and two smaller vials are the safer way to buy it if the work runs in separate sessions and you would rather not hold a reconstituted solution for long. The concentration arithmetic for either size is at /tools/peptide-calculator/.

Storage follows the general rule for lyophilised peptides. Keep unopened vials refrigerated, or frozen for storage beyond a few months, in the dark and dry. Let a cold vial reach room temperature before opening it so that condensation does not form on the powder. After reconstitution, keep the solution at 2 to 8 C, do not freeze and thaw it repeatedly, and discard any solution that turns cloudy or shows particulates.

The diluent caution is specific to this molecule and to its lipidated relatives. Reconstitute with bacteriostatic water, or sterile water where a preservative-free vehicle is required by the method, and nothing else. Sodium-chloride diluents are not a validated solvent for this lipidated, acetate-buffered material and have been associated with cloudiness and precipitation on addition. A vial that goes cloudy on reconstitution should be discarded, not filtered and used. Add the diluent slowly down the inside wall of the vial and swirl gently; do not shake. At low working concentrations, lipidated peptides adsorb to plain polypropylene, so low-binding tubes and tips are worth using. Diluent volume, concentration and everything downstream of that is protocol, and it is on the protocol page rather than here: the reported protocol, including a concentration table computed from the vial mass we actually ship, is at /protocols/retatrutide-20mg/.

Reading the certificate for a retatrutide lot

Pure North commissioned Canada Peptide Testing, as a paying client. Published reports for this product: 20 mg vial, CPT-RETA-20-091226 (99.31% area purity; 20.24 mg measured content). Read the original documents at /lab-results/.

The published Canada Peptide Testing reports measure RP-HPLC-UV area purity and content. They do not include mass-spectrometry identity, endotoxin, water content or a related-substances breakdown. These results do not establish clinical safety or regulatory approval.

Match the compound, vial size and printed lot to the report before relying on it. An assigned reference is not automatically the batch code on a vial. If they differ, email purenorthpeptides@proton.me with the vial details and report number to confirm the connection. Do not extend a report to another size or lot.

Regulatory status in Canada

Retatrutide is supplied in Canada as a research-use-only laboratory reference standard. It is not approved by Health Canada for any use in people, it carries no Drug Identification Number or Natural Product Number, and it is not an authorised therapeutic anywhere; it remains an investigational compound in ongoing trials. That is the category the material sits in, and it constrains what we can say: no health claims, no outcomes. The trial results summarised on this page are a description of the published literature, not a description of the product, and the schedules on the protocol page are a summary of what that literature reports rather than a recommendation.

Every order requires a research-use declaration at checkout confirming that the material is for laboratory research. That is the buyer's side of the same category.

The clinical product is a different object from the research standard even where the sequence is identical: it is manufactured, formulated, filled and released under a regulatory framework that research reference material is not, and it is given under medical supervision inside a trial protocol. This page is general information, not legal or medical advice. Classification is compound-specific and can change, Health Canada is the authority, and our overview of the research-use framework is at /library/are-research-peptides-legal-in-canada/.

Frequently asked

What is retatrutide?

Retatrutide (LY3437943) is a synthetic 39-residue peptide that activates three receptors: GLP-1R, GIPR and the glucagon receptor. A C20 fatty-diacid arm binds serum albumin and gives it a functional half-life of days, which is why its trial programme used a once-weekly schedule. It is an investigational compound developed by Eli Lilly, and it is supplied here as a research-use-only laboratory reference standard.

How is retatrutide different from tirzepatide?

Tirzepatide is a dual agonist at GIPR and GLP-1R; retatrutide adds glucagon-receptor activity as a third arm. Otherwise the two are close relatives: the same length, the same GIP-derived scaffold and the same class of C20 lipidation. The glucagon arm is hypothesised to add effects on energy expenditure and hepatic fat, and the trial literature discusses a heart-rate observation that the glucagon arm may compound. Semaglutide, a single GLP-1R agonist, completes the mono, dual and triple comparison set.

Is retatrutide approved in Canada?

No. Retatrutide is an investigational compound with no marketing authorisation from Health Canada or any other regulator as of the date on this page. It carries no DIN or NPN. It is sold here as a research-use-only laboratory reference standard, a category distinct from products approved for human use, and buyers sign a research-use declaration at checkout. Confirm your own position with Health Canada rather than with a supplier's website.

Which retatrutide vial size should a lab choose?

We stock 10 mg and 20 mg vials. The decision is about how much material the study needs and the price per milligram, which is printed on the product page; the larger vial is cheaper per milligram, and the smaller one suits work that runs in separate sessions where you would rather not hold reconstituted solution for long. Use /tools/peptide-calculator/ to work out concentration for either size. Nothing about the choice of vial implies anything about amounts used in an experiment, which is protocol and is not covered here.

How should retatrutide be stored and reconstituted?

Keep lyophilised vials refrigerated, or frozen for longer storage, dry and out of light, and let a cold vial warm to room temperature before opening it. Reconstitute with bacteriostatic water, or sterile water where the method requires a preservative-free vehicle, and never with a sodium-chloride diluent: saline is not a validated solvent for this lipidated, acetate-buffered material and has been associated with cloudiness and precipitation. Discard any vial that clouds. Store reconstituted solution at 2 to 8 C and avoid repeated freeze-thaw. Volumes and concentrations are on the protocol page, not here.

Referenced in our catalog

Related library entries