Retatrutide (LY3437943) is a synthetic, fatty-acid-conjugated long-acting peptide developed by Eli Lilly as the first-in-class triple agonist of the GIP, GLP-1 and glucagon receptors. Where dual agonists such as Tirzepatide drive weight management primarily through appetite and insulin pathways, the addition of glucagon-receptor agonism in Retatrutide is studied as a mechanism for increasing resting energy expenditure — making it the most metabolically active research compound in the incretin class to date.
Retatrutide (LY3437943) is a first-in-class, once-weekly triple agonist of the GIP, GLP-1 and glucagon receptors developed by Eli Lilly. In the Phase 2 TRIUMPH-1 obesity trial (NEJM, 2023) it produced up to 24.2% mean body-weight reduction at 48 weeks — the largest effect reported for any incretin-class compound — and in a Phase 2a MASLD study (Nature Medicine, 2024) it reduced liver fat by roughly 82%. Its differentiator is the glucagon arm, which raises resting energy expenditure, so it lowers energy intake and increases energy expenditure at once. It has a ~6-day half-life, is supplied as a lyophilised powder reconstituted with bacteriostatic water, and remains an investigational compound for in vitro research use only — not approved for human use. Research-grade Retatrutide is available from RS Bio Labs in eight strengths (5–60 mg) at ≥99% purity; see the buy Retatrutide UK page.
Retatrutide, known by its developer code LY3437943, is a synthetic 39-amino-acid peptide engineered by Eli Lilly to activate three distinct metabolic receptors simultaneously: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R) and the glucagon receptor (GCGR). Because it engages all three, it is described in the literature as a "triple agonist" or "GGG tri-agonist", placing it one generation beyond the dual GIP/GLP-1 agonist Tirzepatide and two beyond the single GLP-1 agonist Semaglutide.
The compound has become the most closely watched molecule in metabolic research because of the sheer magnitude of effect it produced in early trials. The unifying idea behind its design is deceptively simple: the GLP-1 and GIP arms reduce energy intake (appetite suppression, slowed gastric emptying, glucose-dependent insulin secretion), while the glucagon arm increases energy expenditure (hepatic lipid mobilisation and a measurable rise in resting metabolic rate). No single- or dual-agonist compound can do both at once, and it is this "energy-in, energy-out" combination that researchers believe underpins Retatrutide's results.
As of 2026 Retatrutide is an investigational compound in Phase 3 development and is not approved by any regulator. RS Bio Labs supplies it strictly as a research reagent for in vitro laboratory work. Researchers looking to buy Retatrutide in the UK can obtain it in eight strengths from 5 mg to 60 mg, each third-party HPLC tested to ≥99% purity.
Retatrutide is the product of a decade-long incretin-engineering programme at Eli Lilly. The scientific lineage runs from native glucagon-like peptide-1 — a gut hormone with a half-life of only a couple of minutes — through long-acting GLP-1 analogues, to the dual GIP/GLP-1 agonist Tirzepatide, and finally to the addition of a third receptor arm in Retatrutide.
The compound's preclinical characterisation was published by Coskun and colleagues in Cell Metabolism in 2022, in a paper titled "LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: from discovery to clinical proof of concept." That work described the molecule's receptor-binding profile — balanced GCGR and GLP-1R activity with comparatively more prominent GIPR activity — and demonstrated in animal models that the triple mechanism produced greater weight loss than dual agonism, without the hyperglycaemia that unopposed glucagon-receptor activation might be expected to cause.
First-in-human Phase 1 studies (ClinicalTrials.gov NCT04881760) established the compound's tolerability and its approximately six-day half-life, confirming that a once-weekly schedule was feasible. These data set the stage for the TRIUMPH Phase 2 programme across obesity, type 2 diabetes and metabolic dysfunction-associated steatotic liver disease (MASLD), and subsequently the Phase 3 TRIUMPH and TRANSCEND trials that are ongoing today.
Retatrutide is a single peptide molecule that binds and activates three structurally related class B G-protein-coupled receptors. Each sits at a different node of the energy-balance network, and the combined agonism produces a metabolic phenotype that is qualitatively distinct from any mono- or dual-agonist compound studied to date. Receptor affinity has been tuned across all three targets to preserve metabolic synergy while limiting off-target effects.
The glucagon arm is the crux of the design. In isolation, glucagon-receptor activation raises blood glucose — an undesirable effect. But when paired with potent GLP-1 and GIP activity, the insulin-promoting arms offset that tendency, allowing the metabolic-rate benefit of glucagon signalling to be harnessed while glycaemic control is maintained or improved. The molecule itself is a fatty-acid-conjugated (C20) long-acting peptide; the fatty-acid tail promotes reversible binding to serum albumin, which is what gives Retatrutide its extended, once-weekly duration of action. For a broader treatment of how these three pathways differ, see our guide to GLP-1 vs GIP vs glucagon agonists.
Retatrutide's clinical evidence base spans obesity, type 2 diabetes and liver disease, and is unusually strong for a compound at this stage. The three landmark Phase 2 readouts are summarised below, followed by the individual trial cards.
| Trial / population | Publication | n | Headline result |
|---|---|---|---|
| Phase 2 · Obesity (TRIUMPH-1) | NEJM, 2023 | 338 | ~17.5% at 24 wk; ~24.2% at 48 wk (12 mg); not plateaued |
| Phase 2 · Type 2 diabetes | Lancet, 2023 | 281 | ~16.9% weight reduction at 24 wk; robust HbA1c reduction |
| Phase 2a · MASLD (liver fat) | Nature Medicine, 2024 | 98 | ~81–82% relative liver-fat reduction; ~93% reached normal liver fat (top dose) |
Retatrutide's defining pharmacokinetic feature is its terminal half-life of approximately six days. This is a direct consequence of the molecule's C20 fatty-acid conjugation, which drives reversible binding to serum albumin. Albumin binding acts as a circulating reservoir, slows renal clearance, and shields the peptide from rapid proteolytic degradation — the same design principle used across the modern long-acting incretin class.
A ~6-day half-life supports a once-weekly research schedule. As with any compound dosed at roughly one half-life intervals, plasma concentrations accumulate over successive administrations and approach steady state after about four to five half-lives — on the order of four to five weeks. This accumulation behaviour is the reason published protocols use gradual dose escalation: titrating upward over several weeks allows tolerance to the gastrointestinal effects to develop as concentrations rise. For a dedicated treatment, see our Retatrutide half-life guide.
| Parameter | Value (approx.) | Note |
|---|---|---|
| Terminal half-life | ~6 days | Fatty-acid/albumin driven |
| Research schedule | Once weekly | ~1 half-life dosing interval |
| Time to steady state | ~4–5 weeks | ~4–5 half-lives |
| Molecular weight | ~4,731 Da | 39-aa peptide + C20 tail |
| Elimination | Proteolysis + renal | Slowed by albumin binding |
As a lyophilised (freeze-dried) powder, Retatrutide is highly stable. Removing water from the formulation dramatically slows the chemical degradation pathways — hydrolysis, deamidation, oxidation and aggregation — that shorten a peptide's shelf life in solution. Sealed, protected from light and kept cool, lyophilised Retatrutide retains its integrity over long periods.
Stability falls once the powder is reconstituted. In solution the peptide becomes susceptible to gradual hydrolysis and to physical stresses such as agitation (which can cause aggregation and foaming) and repeated freeze–thaw cycles (which mechanically damage peptide structure). Heat and light accelerate degradation in both states. In practical terms this means the lyophilised vial is the storage-stable form, and the reconstituted solution should be treated as a shorter-lived working stock — see Storage and Reconstitution below.
Correct storage preserves both the mass and the biological integrity of the compound. The guidance below reflects standard best practice for lyophilised research peptides; our full Retatrutide storage guide covers it in more depth.
| State | Temperature | Approx. shelf life | Notes |
|---|---|---|---|
| Lyophilised | −20 °C (freezer) | Months to years | Best long-term option; protect from light |
| Lyophilised | 2–8 °C (fridge) | Weeks to months | Fine for medium-term storage |
| Lyophilised | Room temperature | Days (transit) | Tolerates brief shipping periods |
| Reconstituted | 2–8 °C (fridge) | ~4–6 weeks | Bacteriostatic water aids stability; keep dark |
| Reconstituted | Frozen | Avoid | Freeze–thaw degrades the peptide |
Label reconstituted vials with the date of preparation, minimise exposure to light and heat, and avoid repeatedly warming and cooling a working stock. Allow refrigerated vials to return toward room temperature before handling to reduce condensation.
The following describes standard laboratory technique for preparing a research stock solution from a lyophilised vial. It is not guidance for human use of any kind. For a fully worked walkthrough with more dilution examples, see reconstituting Retatrutide.
Reconstitution simply means dissolving the freeze-dried powder into solution. The standard diluent for a multi-use research vial is bacteriostatic water (water with 0.9% benzyl alcohol, which suppresses microbial growth). You will also need a sterile syringe and alcohol wipes. Technique matters: wipe the vial stopper, draw the chosen volume of bacteriostatic water, and add it slowly down the inner glass wall rather than squirting it directly onto the powder. Let the peptide dissolve, swirl gently, and never shake — agitation causes foaming and can damage the peptide.
Concentration is simply the mass of peptide divided by the volume of water added. Adding more water gives a lower concentration:
| Vial | Bacteriostatic water added | Resulting concentration |
|---|---|---|
| 10 mg | 1 ml | 10 mg/ml |
| 10 mg | 2 ml | 5 mg/ml |
| 30 mg | 3 ml | 10 mg/ml |
| 30 mg | 2 ml | 15 mg/ml |
| 60 mg | 3 ml | 20 mg/ml |
Once reconstituted, store the vial refrigerated at 2–8 °C and use it within a few weeks (see Storage). Maintain sterility throughout and label the vial with the concentration and preparation date.
Retatrutide sits at the top of the incretin "agonism ladder." The table below places it against the other most-studied metabolic research peptides by receptor targets and headline trial effect. For the mechanistic detail behind these classes, see GLP-1 vs GIP vs glucagon; for a direct head-to-head, see Retatrutide vs Tirzepatide.
| Compound | Receptor targets | Class | Peak trial weight change | Half-life |
|---|---|---|---|---|
| Retatrutide | GIP + GLP-1 + Glucagon | Triple | ~24.2% (Ph2, 48 wk) | ~6 days |
| Tirzepatide | GIP + GLP-1 | Dual | ~22.5% (Ph3, 72 wk) | ~5 days |
| Semaglutide | GLP-1 | Mono | ~15% (Ph3, 68 wk) | ~7 days |
| Survodutide | GLP-1 + Glucagon | Dual | MASH-focused Ph2 | ~6 days |
| Mazdutide | GLP-1 + Glucagon | Dual | ~14% (Ph3, 48 wk) | Weekly |
The pattern is consistent: engaging more complementary receptor systems has raised the ceiling on effect size at each step, and Retatrutide's addition of the glucagon "energy-out" arm is what places it ahead of the dual agonists in cross-trial comparisons. To compare live pricing and strengths, see the buy Retatrutide UK, Tirzepatide and Semaglutide pages.
The findings above are drawn from the following peer-reviewed publications and trial registrations. Links open on the publisher or registry site.