Retatrutide

Retatrutide, development code LY3437943, is an investigational peptide that acts as an agonist at three receptors at once: the glucose-dependent insulinotropic polypeptide (GIP) receptor, the glucagon-like peptide-1 (GLP-1) receptor and the glucagon receptor. It is in Phase 3 clinical trials and is not approved as a medicine anywhere in the world. This page lists every brand of retatrutide stocked at Free Muscle and compares them on form, pack size and batch testing, alongside what the published trials actually report.

What is retatrutide?

Retatrutide was engineered by Eli Lilly as a single 39-amino-acid peptide, based on the GIP sequence and modified with a fatty acid chain so that it binds albumin and lasts long enough in the body for once-weekly dosing. Its distinguishing feature is glucagon receptor activity on top of the GIP and GLP-1 agonism found in existing licensed medicines.

Its status needs stating precisely. Retatrutide is an investigational medicine. Phase 2 trials in obesity and in type 2 diabetes were published in 2023, and Phase 3 trials (the TRIUMPH programme) are ongoing. It has no marketing authorisation from the MHRA, the EMA or the FDA, so it cannot be prescribed outside a clinical trial, and the long-term safety picture will not be known until the Phase 3 data are complete. Retatrutide is not currently named on the WADA Prohibited List, but tested athletes should check the current list themselves rather than rely on this page.

What the published research reports

The Phase 2 obesity trial randomised 338 adults with obesity or overweight to once-weekly subcutaneous retatrutide at several dose levels or placebo for 48 weeks. Body weight fell in a dose-related way, by up to about 24 per cent at 48 weeks in the highest-dose group compared with about 2 per cent on placebo. The most common adverse events were gastrointestinal (nausea, diarrhoea, vomiting, constipation), they were dose-related, and heart rate rose in a dose-dependent way, peaking at 24 weeks (Jastreboff 2023). The parallel Phase 2 trial in 281 people with type 2 diabetes reported reductions in HbA1c and body weight versus placebo and versus dulaglutide over 36 weeks, again with gastrointestinal adverse events in about a third of retatrutide participants (Rosenstock 2023). The discovery paper describes the molecule's design and the Phase 1 proof-of-concept data in humans (Coskun 2022).

Two limitations dominate. Every trial used subcutaneous injection under clinical supervision with gradual dose escalation, and peptides of this size are largely broken down in the digestive tract, so nothing in the literature supports the idea that a capsule reproduces these results. And the trials were funded and run by the manufacturer, with Phase 3 outcomes and long-term safety still unreported. None of the above is a claim about any product on this page.

• Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526. PMID: 37366315.
• Rosenstock J, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial conducted in the USA. Lancet. 2023;402(10401):529-544. PMID: 37385280.
• Coskun T, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metab. 2022;34(9):1234-1247.e9. PMID: 35985340.

Brands of retatrutide we stock

• Free Muscle Retatrutide Research Peptide. Free Muscle brand. Form, strength and pack size are stated on the product page. Every Free Muscle batch is assayed by Janoshik Analytical, an independent laboratory, and the certificate of analysis for the batch you hold is sent by email on request.

Retatrutide is currently stocked from Free Muscle only. If other brands are added they will be listed here with the same information so that the comparison stays consistent across the shop.

How to compare them

Where a strength per unit is stated, multiply it by the units in the pack to get total compound, and divide the price by that number for cost per milligram. With a peptide like this, check the form and route stated on the listing, because an injectable vial and a capsule are not interchangeable and the published trials used only one of them. Above all, ask for the batch certificate: for a compound this new, a certificate that confirms identity by mass spectrometry and states purity is the only evidence that the pack contains what the label says.

Batch testing and certificates of analysis

A certificate of analysis (COA) is a laboratory report tied to one manufactured batch. It records the identity of the compound tested (usually by HPLC or LC-MS against a reference standard) and the purity found, and it carries the batch number, the test date and the name of the laboratory that ran the assay. Every Free Muscle-brand batch is assayed by Janoshik Analytical, an independent laboratory, and we will send the certificate for the batch you hold if you email us with the batch number from the label. For products from the other brands we stock (iMuscle, Yakka Strength, Thoroughbred Labs and BioLab), none of which currently sells retatrutide, a COA is shown only where the manufacturer has supplied one for the specific batch we hold, and we leave the field blank rather than imply a test that does not exist.

A generic PDF with no batch number, no date or no named laboratory is not a batch certificate, whatever it is called. Match the batch number on the certificate to the number on your pack before relying on it. Our lab tests page explains what to look for.

Legal status in the UK

Retatrutide is not a licensed medicine in the UK and has no MHRA marketing authorisation, nor does it hold approval from the EMA or FDA. It is an investigational medicine available to patients only within clinical trials. It is not an authorised food supplement and may not be sold for human consumption. It is supplied here for laboratory research to adults aged 18 and over. It is not a controlled drug. Anyone seeking treatment for weight or blood glucose should speak to a doctor, since this is not a treatment and licensed options with clinical supervision exist. For background on how research peptides sit within UK law, see our SARMs versus peptides guide.

Storage and handling

Keep the container sealed, in a cool, dry place away from direct light, and return it there each time the container has been opened. Do not decant into unlabelled containers. Keep out of the reach of children and animals. Handle in line with ordinary laboratory good practice, with gloves and a clean work surface, and dispose of unwanted material through an appropriate waste route rather than down the sink.

Delivery and returns

Free Muscle is a UK company (FreeMuscle LTD, Scunthorpe). Every order ships with free tracked UK delivery in plain packaging with nothing on the outside to say what is inside. Orders placed before 2pm Monday to Friday are processed the same working day. EU delivery takes three to seven working days. Unopened items can be returned within 30 days. Customers must be 18 or over. Full details are on our shipping page and our guarantee page.

Related reading

• SLU-PP-332 and 5-Amino-1MQ: what the mouse data show
• Do oral peptides actually work? The honest answer
• What is Janoshik Analytical? Third-party testing explained
• How to read a certificate of analysis
• Metabolic support, all peptides, 5-Amino-1MQ and Ipamorelin

Retatrutide (LY3437943) is supplied for laboratory research use only. It is not a licensed medicine, not a food supplement and not for human consumption. Nothing on this page has been evaluated by the MHRA, EMA or FDA, and nothing on this page is medical, health or dosing advice. Sales are restricted to adults aged 18 and over.