Key takeaways

Phase-2 TRIUMPH-1 trial reported mean weight reduction of −24.2% at 48 weeks on the 12mg dose — exceeding any approved compound.
Triple-receptor mechanism adds glucagon-receptor activation to the GIP/GLP-1 framework — increasing energy expenditure as well as suppressing intake.
Phase-3 TRIUMPH program ongoing; obesity readout expected 2026–2027.
No regulatory approval anywhere as of May 2026. Not legally available outside trials.
Side-effect profile broadly resembles tirzepatide; long-term safety data are limited.

What it is

Retatrutide is a single-molecule triple-receptor agonist that activates GLP-1, GIP, and glucagon receptors. It is structurally derived from the GIP backbone, with sequence modifications that introduce GLP-1 and glucagon agonist activity. Like tirzepatide, it is engineered for once-weekly subcutaneous administration via reversible albumin binding.

It is currently investigational. Eli Lilly's phase-3 TRIUMPH program is testing it in obesity, type-2 diabetes, NASH, and obstructive sleep apnoea. No approval exists in any jurisdiction as of May 2026.

How it works

Retatrutide stacks three incretin-family pathways. The GIP and GLP-1 actions resemble tirzepatide. Adding glucagon-receptor agonism increases hepatic energy turnover and contributes to weight reduction via increased energy expenditure — not just appetite suppression.

Step 1

Subcutaneous injection

Weekly dose absorbed; albumin-bound for slow release.

Step 2

Triple receptor binding

GLP-1 + GIP + glucagon receptors activated simultaneously.

Step 3

Metabolic effect

Insulin sensitisation, satiety, and increased energy expenditure.

Step 4

Body composition

Substantial fat-mass reduction; lean mass preserved in early data.

The glucagon component is the novel element. Activation increases hepatic glucose output transiently — but the dominant effect at therapeutic doses is increased basal metabolic rate, contributing to weight reduction beyond what GLP-1/GIP can achieve alone.

Benefits

Weight reduction (medium confidence — phase-2 only)

TRIUMPH-1 (n=338, 48 weeks) reported mean body-weight reductions of −8.7%, −17.1%, and −24.2% at 4mg, 8mg, and 12mg respectively. These are the largest reductions reported for any pharmacologic obesity intervention. Data are phase-2 only — phase-3 confirmation is required before high-confidence framing is appropriate.

Glycaemic control (medium confidence)

In type-2 diabetics, phase-2 data showed HbA1c reductions of approximately −2.0% across the higher dose tiers. Effect sizes are similar to tirzepatide. Definitive head-to-head data are not yet published.

What the evidence does not yet show

Phase-3 efficacy and safety data, long-term durability beyond 48 weeks, cardiovascular outcomes, and head-to-head comparisons with tirzepatide are all pending. Confidence ratings will rise once that evidence is published. Until then, this profile reflects an extrapolation from phase-2 data only.

Research summary

Triple hormone receptor agonist retatrutide for obesity (TRIUMPH-1 phase-2)

2023

Phase-2 RCT, double-blind, placebo-controlled·n = 338

Mean weight reduction at 48 weeks: −24.2% on 12mg vs −2.1% on placebo. 100% of participants on 8mg or 12mg achieved ≥5% reduction; 83% on 12mg achieved ≥20%.

N Engl J Med · doi:10.1056/NEJMoa2301972

Retatrutide in patients with type-2 diabetes (phase-2)

2023

Phase-2 RCT, dose-finding·n = 281

HbA1c reductions of up to −2.02% at the 12mg dose over 36 weeks. Concomitant weight reduction of up to −16.9%. Side-effect profile dominated by GI symptoms.

Lancet · doi:10.1016/S0140-6736(23)01053-X

TRIUMPH-3 (phase-3 obesity & cardiovascular endpoints)

Ongoing

Phase-3 RCT, multi-endpoint·n = ~6,000

Phase-3 weight-management readout expected 2026–2027. Cardiovascular outcomes secondary endpoint. Will provide first regulatory-grade evidence.

ClinicalTrials.gov · NCT05882045

Dosage & administration

Retatrutide is investigational. The dose schedule below reflects the protocol used in the TRIUMPH-1 phase-2 trial; it is provided for educational reference only. The compound is not legally available for use outside clinical trials.

Source: TRIUMPH-1 phase-2 trial protocol. Not regulatory dosing — no MHRA SmPC exists.
Phase	Dose	Duration	Notes
Initiation	2.0 mg / week	4 weeks	Trial initiation dose
Step 1	4.0 mg / week	4+ weeks	Lowest study dose
Step 2	8.0 mg / week	4+ weeks	Mid-range study dose
Maximum	12.0 mg / week	—	Highest dose tested

Side effects & safety

Phase-2 safety data resemble tirzepatide — GI-dominated and dose-dependent. Long-term and rare-event data are limited by trial sample size.

Effect	Frequency	Severity
Nausea	30–35%	Mild–moderate
Diarrhoea	15–22%	Mild
Constipation	8–12%	Mild
Vomiting	6–14%	Mild–moderate
Heart rate increase	~7 bpm	Monitor
Acute pancreatitis	<0.5%	Serious — trial-only data

Contraindications: No regulatory contraindication list exists. Trial exclusions: personal/family history of medullary thyroid carcinoma or MEN-2; pancreatitis; severe gastroparesis; pregnancy.

UK legal status

Investigational — not legally available outside trials

Retatrutide has no marketing authorisation in any jurisdiction as of May 2026. UK supply outside an authorised clinical trial is illegal under the Human Medicines Regulations 2012 and Medicines and Medical Devices Act 2021. Any product marketed under the name "retatrutide" through unregulated channels is not the trial compound, has no quality assurance, and cannot be assumed to contain the labelled ingredient.

Summary

Retatrutide is the most promising next-generation obesity peptide in development. Phase-2 efficacy signals exceed every approved compound — but phase-3 confirmation is essential before treating those numbers as established. UK readers should be aware that the compound is not legally available outside clinical trials, and any "retatrutide" sold via unregulated channels has no verified identity or quality. We will revise this profile when phase-3 readouts are published.

References (5)

Jastreboff AM, et al. Triple-hormone-receptor agonist retatrutide for obesity (phase 2). N Engl J Med. 2023;389(6):514-526.
Rosenstock J, et al. Retatrutide in adults with type-2 diabetes (phase 2). Lancet. 2023;402(10401):529-544.
Coskun T, et al. Pharmacology of LY3437943: a unimolecular GIP/GLP-1/glucagon receptor agonist. Cell Metab. 2022;34(9):1234-1247.
ClinicalTrials.gov. TRIUMPH program. Multiple identifiers: NCT05882045 (TRIUMPH-3), NCT05929066 (TRIUMPH-4).
European Medicines Agency. Note: as of May 2026, retatrutide has no marketing authorisation application under EMA review.

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