Retatrutide is a 39-amino acid synthetic peptide developed by Eli Lilly under the research code LY3437943. It is the first compound to simultaneously activate three metabolic hormone receptors — GLP-1, GIP, and glucagon — and is currently completing the Phase 3 TRIUMPH clinical programme, with pivotal trial results published in May 2026. It represents the next generation of incretin-based research compounds after semaglutide and tirzepatide.
This page is a research reference for the retatrutide peptide: mechanism of action, clinical trial data through July 2026, comparison with other compounds in the GLP-1 class, safety profile, and the current regulatory timeline for UK availability.
What Is Retatrutide?
Retatrutide is a synthetic peptide — a chain of 39 amino acids engineered to bind with high affinity to the receptors of three gut-derived and metabolic hormones simultaneously. This distinguishes it structurally and pharmacologically from all currently approved compounds in the incretin class.
- Semaglutide (Wegovy, Ozempic) is a GLP-1 mono-agonist: one receptor target
- Tirzepatide (Mounjaro, Zepbound) is a GLP-1/GIP dual agonist: two receptor targets
- Retatrutide is a GLP-1/GIP/glucagon triple agonist: three receptor targets simultaneously
Each additional receptor target adds a mechanistically distinct pathway, which is why preclinical research and clinical trials have consistently shown retatrutide producing greater metabolic effects than either mono- or dual-agonist comparators. The compound is administered once weekly by subcutaneous injection, with a dose escalation schedule beginning at 2mg and progressing to a maximum of 12mg over 20 weeks.
The Triple-Agonist Mechanism: What Each Receptor Does
Understanding why retatrutide is being studied as a more potent compound than its predecessors requires understanding what each of the three receptor pathways actually contributes.
GLP-1 Receptor Agonism
Glucagon-like peptide-1 is released from L-cells in the gut after eating. GLP-1 receptor agonism is the shared mechanism across the entire class — it drives central appetite suppression through hypothalamic pathways, slows gastric emptying to prolong satiety, and stimulates glucose-dependent insulin secretion. This is the mechanism responsible for reduced caloric intake across all three compounds.
For a detailed comparison of how different GLP-1 agonists differ in their mechanisms and research applications, see our overview of the GLP-1 agonist class.
GIP Receptor Agonism
Glucose-dependent insulinotropic polypeptide works synergistically with GLP-1. In tirzepatide's SURMOUNT and SURPASS trials, the addition of GIP agonism was the primary reason tirzepatide outperformed semaglutide head-to-head on weight loss. GIP agonism improves insulin sensitivity in peripheral tissues, modulates fat deposition in adipose tissue, and — critically — reduces the gastrointestinal side effects that GLP-1 agonism alone produces.
This tolerability improvement is one reason tirzepatide's dose discontinuation rates were lower than semaglutide despite greater efficacy.
Glucagon Receptor Agonism — The Key Differentiator
This is the pathway that makes retatrutide mechanistically distinct. Glucagon is a catabolic hormone that, at physiological levels, promotes fat breakdown (lipolysis), increases hepatic glucose output, and — importantly — raises basal metabolic rate through a direct thermogenic effect.
The scientific concern with glucagon agonism has historically been glucose dysregulation: glucagon raises blood sugar. Eli Lilly's design addresses this by pairing glucagon agonism with potent GLP-1 and GIP activity, which suppresses glucagon's glycaemic effects through compensatory insulin stimulation. What remains is the glucagon signal the body actually responds to: accelerated fat oxidation, increased energy expenditure, and enhanced liver fat clearance.
In plain terms: GLP-1/GIP reduces caloric intake. Glucagon increases caloric burn. The combination produces substantially greater net metabolic effect than either mechanism alone — and this is what the TRIUMPH Phase 3 programme has now validated in human trials at scale.
Phase 2 Clinical Trial Results
The foundational efficacy data for retatrutide was published in the New England Journal of Medicine in 2023 (NCT04881760). The Phase 2 trial enrolled 338 adults with obesity or overweight, randomised to weekly retatrutide at doses of 1, 2, 4, 8, or 12mg, or placebo, over 48 weeks.
Key findings at 48 weeks:
| Dose | Average weight loss |
|---|---|
| 4mg | 17.5% of body weight |
| 8mg | 22.8% of body weight |
| 12mg | 24.2% of body weight |
| Placebo | 2.1% of body weight |
At the 12mg dose, 26% of participants lost at least 30% of their initial body weight — a threshold that historically has only been achieved through bariatric surgery. The Phase 2 data positioned retatrutide as the highest-efficacy compound in the incretin class at the time of publication and initiated the Phase 3 TRIUMPH programme.
For a deeper analysis of what the Phase 2 data revealed about retatrutide's triple-agonist properties, see our earlier review: Retatrutide — a promising triple agonist in human trials for obesity and type 2 diabetes.
Phase 3 TRIUMPH Programme: 2025–2026 Results
The TRIUMPH programme is Eli Lilly's Phase 3 clinical programme for retatrutide, comprising eight trials across obesity, type 2 diabetes, cardiovascular disease and other metabolic conditions. As of July 2026, three pivotal trial results have been made public.
TRIUMPH-4: Obesity and Knee Osteoarthritis (December 2025)
The first Phase 3 readout evaluated retatrutide at 9mg and 12mg in adults with obesity or overweight and knee osteoarthritis. This trial was notable for investigating the compound's effects on both metabolic and musculoskeletal outcomes simultaneously.
Weight loss results:
- 9mg dose: approximately 23–25% average weight loss
- 12mg dose: average weight loss producing a mean of 71.2 lbs (32.3kg) reduction per participant
Osteoarthritis outcomes:
- WOMAC pain scores (the standard measure of arthritis pain severity) reduced by up to 4.5 points, representing a 75.8% reduction from baseline
- More than 1 in 8 participants on retatrutide were completely free from knee pain at the end of the trial
- Significant improvements in physical function measures alongside pain reduction
Safety signal — dysesthesia: A notable finding in TRIUMPH-4 was the rate of dysesthesia — an abnormal sensation of touch or skin sensitivity. This occurred in 8.8% of participants on the 9mg dose and 20.9% on the 12mg dose, compared with 0.7% in the placebo group. Lilly characterised these events as generally mild and rarely leading to treatment discontinuation. This side effect had been observed at lower rates in Phase 2 (7% at 12mg) and appears to be a class-related effect of glucagon receptor agonism.
Overall discontinuation rates due to adverse events were 12.2% (9mg) and 18.2% (12mg) versus 4.0% on placebo. For participants with baseline BMI ≥35, discontinuation rates were lower (8.8% and 12.1% respectively), suggesting that the tolerability profile varies by baseline metabolic status.
TRANSCEND-T2D-1: Type 2 Diabetes (March 2026)
Eli Lilly announced positive topline results from the first Phase 3 diabetes trial in March 2026. Retatrutide met its primary endpoints, demonstrating significant HbA1c reduction alongside substantial body weight loss in adults with type 2 diabetes. Full peer-reviewed data are expected at a medical conference later in 2026.
TRIUMPH-1: Pivotal Obesity Trial (May 2026)
TRIUMPH-1 is the primary Phase 3 weight management trial — the direct equivalent of the SURMOUNT-1 trial that supported tirzepatide's approval and the STEP-1 trial that supported semaglutide. The trial enrolled 2,339 participants with obesity or overweight and at least one weight-related comorbidity, but without diabetes, randomised to retatrutide 4mg, 9mg, 12mg, or placebo.
Results at 80 weeks:
| Dose | Average weight loss | Average lbs lost |
|---|---|---|
| 4mg | 19.0% | 47.2 lbs |
| 9mg | 25.9% | 64.4 lbs |
| 12mg | 28.3% | 70.3 lbs |
| Placebo | 3.9% | — |
All doses met primary and key secondary endpoints. Notably, 45.3% of participants on the 12mg dose achieved at least 30% body weight reduction — a threshold historically associated with bariatric surgery — and 65.3% reduced their BMI below 30 kg/m². In a pre-specified 104-week extension for the highest-BMI participants, weight loss on the 12mg dose reached 30.3%, averaging 85.0 lbs lost from a baseline of 268.3 lbs.
Cardiometabolic secondary endpoints: Beyond weight loss, TRIUMPH-1 demonstrated significant improvements across cardiovascular risk factors including waist circumference, triglyceride levels, systolic blood pressure, non-HDL cholesterol, and high-sensitivity C-reactive protein (hsCRP). Specific numerical data on these secondary endpoints are expected in the peer-reviewed publication.
Regulatory timeline following TRIUMPH-1: Eli Lilly's New Drug Application to the FDA is expected in the Q4 2026 to Q1 2027 window. Given standard FDA review timelines of 10–12 months, approval in the United States is projected for late 2027 to 2028. MHRA approval in the UK is expected to follow a similar or slightly later timeline.
Comparing Retatrutide to Semaglutide and Tirzepatide
The efficacy data across the three generations of GLP-1-class compounds now allows a direct comparison on a like-for-like basis.
| Semaglutide (Wegovy) | Tirzepatide (Mounjaro) | Retatrutide | |
|---|---|---|---|
| Receptor targets | GLP-1 | GLP-1 + GIP | GLP-1 + GIP + Glucagon |
| Phase 3 peak weight loss | ~15% at 68 wks (STEP-1) | 22.5% at 72 wks (SURMOUNT-1) | 28.3% at 80 wks / 30.3% at 104 wks (TRIUMPH-1) |
| Phase 3 status | Approved (UK) | Approved (UK) | Phase 3 complete, NDA pending |
| UK availability | Available | Available | Expected 2027–2028 |
| Nausea rate (highest dose) | ~44% | ~33% | 42.4% |
| Discontinuation rate (AEs, highest dose) | ~7% | ~8% | 11.3% |
The TRIUMPH-1 data confirms what Phase 2 suggested: each additional receptor target produces meaningful incremental efficacy. The tradeoff is a modestly higher discontinuation rate at the 12mg dose, primarily driven by gastrointestinal effects and dysesthesia, compared with tirzepatide at equivalent trial stages.
Cardiovascular Research: The TRIUMPH-CVD Programme
Beyond weight loss and diabetes, retatrutide is being evaluated for direct cardiovascular outcomes in the TRIUMPH-CVD programme. This is a large outcomes trial studying whether retatrutide reduces the incidence of major adverse cardiovascular events (MACE) — the standard endpoint that established cardiovascular benefit for semaglutide (PIONEER-6, SUSTAIN-6) and forms part of the basis for expanding GLP-1 class use.
Results from TRIUMPH-CVD are not yet available and are expected beyond 2027. However, the secondary cardiovascular endpoint data from TRIUMPH-1 (reductions in blood pressure, triglycerides, hsCRP and non-HDL cholesterol) provide mechanistic support for a cardiovascular benefit signal.
Beyond metabolic and cardiovascular research, retatrutide is also being studied in oncological contexts.
Researchers have explored whether GLP-1 class triple agonists may have a role in cancer research — for an overview of the current evidence, see our article on whether retatrutide can help fight pancreatic cancer.
Side Effects and Safety Profile
The retatrutide safety profile observed across Phase 2 and Phase 3 trials to date shares characteristics with the broader GLP-1 class, with one novel signal.
Common side effects (TRIUMPH-1 data, by dose):
| Side effect | 4mg | 9mg | 12mg | Placebo |
|---|---|---|---|---|
| Nausea | 28.6% | 38.4% | 42.4% | 14.8% |
| Diarrhoea | 25.2% | 34.1% | 32.0% | 13.5% |
| Constipation | 23.8% | 25.9% | 26.1% | 10.9% |
| Vomiting | 10.6% | 22.8% | 25.3% | 4.8% |
These gastrointestinal effects are typical of GLP-1 receptor agonism and are managed through gradual dose escalation. The 4mg dose notably had a lower discontinuation rate due to adverse events (4.1%) than even the placebo group (4.9%), suggesting the lowest dose is well-tolerated. Discontinuation rates rose to 6.9% at 9mg and 11.3% at 12mg.
Dysesthesia: Abnormal skin sensation (tingling, altered touch sensitivity) was observed in TRIUMPH-4, the knee osteoarthritis trial, at rates of 8.8% (9mg) and 20.9% (12mg) versus 0.7% on placebo. In TRIUMPH-1, dysesthesia was noted among adverse events at a lower rate, characterised as mild to moderate. Specific percentages from TRIUMPH-1 are expected in the full peer-reviewed publication. This side effect appears to be related to glucagon receptor activity and represents the most clinically novel signal in the retatrutide safety profile.
UK Regulatory Status and Availability
As of July 2026, retatrutide is not approved for any therapeutic use in the United Kingdom. The MHRA has not reviewed a marketing authorisation application for retatrutide, as Eli Lilly's NDA submission to the FDA is itself not expected until Q4 2026 at the earliest.
Expected timeline:
- Q4 2026 – Q1 2027: FDA NDA submission (United States)
- 2027: FDA review period
- 2027–2028: Expected FDA approval (United States)
- 2028 and beyond: MHRA application and UK approval (estimated)
Retatrutide is available for laboratory research purposes through licensed research peptide suppliers. Regenpeptides supplies retatrutide for research purposes only, in lyophilised form for reconstitution. All products are supplied with certificates of analysis confirming purity.
Frequently Asked Questions
What is retatrutide made of?
Retatrutide is a synthetic peptide — a chain of 39 amino acids — engineered to bind to three hormone receptors (GLP-1, GIP and glucagon) simultaneously. It is not derived from human or animal sources; it is manufactured through solid-phase peptide synthesis.
How does retatrutide differ from Mounjaro (tirzepatide)?
Mounjaro targets two receptors (GLP-1 and GIP). Retatrutide adds a third — the glucagon receptor — which contributes a direct energy expenditure signal and enhanced fat oxidation on top of tirzepatide's dual mechanism. TRIUMPH-1 data show approximately 2–3 percentage points greater weight loss for retatrutide vs tirzepatide's SURMOUNT-1 results at comparable timepoints.
What were the TRIUMPH-1 results?
TRIUMPH-1, published by Eli Lilly in May 2026, showed average weight loss of 17.6% (4mg), 23.7% (9mg), and 25.0% (12mg) at 80 weeks. In a 104-week extension group, the 12mg dose achieved 30.3% average weight loss — a level previously associated with bariatric surgery.
Is retatrutide available in the UK?
Not as an approved medication. Regulatory approval is not expected before 2027–2028. Retatrutide is available as a research compound for laboratory use from licensed suppliers.
What is dysesthesia and why does retatrutide cause it?
Dysesthesia is an abnormal skin sensation — tingling, altered sensitivity, or an unusual sense of touch. It occurred in 8.8–20.9% of TRIUMPH-4 participants depending on dose, compared with 0.7% on placebo. The mechanism is not yet fully established. Most cases were mild and did not lead to treatment discontinuation.
How does retatrutide compare to cagrilintide?
Cagrilintide is a long-acting amylin analogue being studied by Novo Nordisk in combination with semaglutide (CagriSema). Rather than targeting the glucagon receptor, cagrilintide works through amylin pathways to reduce food intake and body weight. CagriSema Phase 3 data showed approximately 22–23% weight loss, positioning it similarly to tirzepatide but below retatrutide's Phase 3 results. The compounds represent different biochemical approaches to the same metabolic problem.
This article is for research and informational purposes only. Retatrutide is not approved for therapeutic use in the United Kingdom or the United States. All Regenpeptides products are supplied exclusively for laboratory research and are not intended for human consumption. Researchers should ensure compliance with all applicable institutional and regulatory guidelines.