Retatrutide
LY3437943, triple GIP/GLP-1/glucagon receptor agonist, triple G agonist, ret, reta
Retatrutide is an investigational peptide that activates three receptors at once: GIP, GLP-1 and glucagon. Adding glucagon receptor activity is intended to raise energy expenditure on top of appetite suppression. Phase 2 trials produced the largest weight reductions yet reported for a drug, but it is not approved anywhere and its phase 3 results have so far been announced only by press release.
Mechanism
Retatrutide is a single synthetic peptide agonist at three class B G-protein-coupled receptors: the GIP receptor, the GLP-1 receptor and the glucagon receptor. The GIP and GLP-1 arms work as they do in tirzepatide, driving glucose-dependent insulin secretion, glucagon suppression at the alpha cell, delayed gastric emptying and central suppression of food intake through hypothalamic and hindbrain circuits.
The glucagon receptor arm is the genuinely new element and it is counterintuitive, because glucagon raises blood glucose. The design logic is that hepatic glucagon receptor activation increases energy expenditure, stimulates lipolysis and hepatic fatty acid oxidation, and reduces liver fat, while the simultaneous incretin activity more than offsets the glycaemic penalty. In effect, the incretin arms suppress energy intake and the glucagon arm raises energy output, attacking both sides of the energy balance equation. That combination is the proposed reason the weight loss exceeds what dual agonists achieve, though no head-to-head trial has tested the proposition. It also introduces risks that pure incretin agonists do not carry, notably a dose-dependent rise in heart rate and the theoretical possibility of unmasking hyperglycaemia if incretin activity is inadequate. The molecule is acylated to bind albumin, giving a half-life of around six days.
What the research shows
The published, peer-reviewed human evidence consists of two phase 2 randomised trials. In the obesity trial, 338 adults were randomised to retatrutide at 1, 4, 8 or 12 mg weekly or placebo for 48 weeks. Mean weight change at 48 weeks was -8.7%, -17.1%, -22.8% and -24.2% across the ascending doses against -2.1% on placebo. Critically, the weight-loss curve had not plateaued at 48 weeks at the higher doses, which is unusual and suggested the ceiling had not been reached. The companion phase 2 trial in 281 people with type 2 diabetes reported HbA1c reductions of up to roughly 2.0 percentage points alongside substantial weight loss, exceeding the dulaglutide active comparator at higher doses.
As of August 2026, the phase 3 TRIUMPH programme has reported topline results by company announcement only. Eli Lilly announced TRIUMPH-1 results on 21 May 2026: in 2,339 participants randomised 1:1:1:1 to 4 mg, 9 mg, 12 mg or placebo, mean weight reduction at 80 weeks was 19.0%, 25.9% and 28.3% across ascending doses against 2.2% on placebo, with 45.3% of the 12 mg group achieving at least 30% weight loss; participants with baseline BMI at or above 35 who entered a study extension reached a mean 30.3% at 104 weeks. Further announcements in July 2026 reported TRIUMPH-2 and TRIUMPH-3 as positive. These figures come from press releases and investor communications, not peer-reviewed publications, and they carry no safety tables, discontinuation breakdowns, subgroup analyses or independent statistical review. They should be treated as provisional until the full trial reports are published. A regulatory submission to the FDA is stated by the sponsor as planned for the first quarter of 2027. Retatrutide is not approved in any jurisdiction. Peptide sold online under this name is entirely unregulated, is not the clinical-grade material used in these trials, and carries no assurance of identity, purity or concentration.
Evidence assessment
Mixed evidence
Two consistent, well-conducted phase 2 randomised placebo-controlled trials are published and both were verified against PubMed, but no phase 3 report has been peer-reviewed or published, no outcome data exist, and the compound is not approved by any regulator. Phase 3 topline announcements are sponsor communications and carry no independent tier weight.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial Mixed evidence
Mean weight change at 48 weeks was -8.7%, -17.1%, -22.8% and -24.2% at 1, 4, 8 and 12 mg respectively versus -2.1% with placebo, with the higher-dose curves not yet plateaued.
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 Mixed evidence
Retatrutide produced clinically meaningful reductions in HbA1c and body weight in type 2 diabetes, with efficacy at higher doses exceeding the dulaglutide active comparator.
TRIUMPH-1 phase 3 trial of retatrutide in obesity (sponsor topline announcement) Limited evidence
Reported mean weight reduction at 80 weeks of 19.0%, 25.9% and 28.3% at 4, 9 and 12 mg versus 2.2% on placebo, with 45.3% of the 12 mg group reaching at least 30% weight loss; participants with baseline BMI 35 or above in the extension reached a mean 30.3% at 104 weeks.
Safety
Gastrointestinal adverse events dominated in phase 2 and were dose-related: nausea, vomiting, diarrhoea and constipation, mostly mild to moderate and concentrated during escalation. Two effects distinguish retatrutide from pure incretin agonists and both plausibly trace to glucagon receptor activation. First, a dose-dependent increase in heart rate was observed, peaking around 24 weeks and partially attenuating thereafter; the long-term cardiovascular significance is unknown and is precisely the kind of question a completed and published cardiovascular outcome trial would need to answer. Second, transient increases in glycaemia are a theoretical concern with glucagon agonism, though this was not a prominent finding in the trials. Skin-related adverse events including cutaneous hypersensitivity were reported at higher frequency than expected. Beyond that, the safety record in the peer-reviewed literature is immature: the longest published randomised exposure is 48 weeks in 338 people, which is not enough to characterise uncommon or delayed harms. Larger and longer phase 3 safety data now exist but have not been published in peer-reviewed form, and press releases do not report them. Anyone reading claims about retatrutide's safety made on the basis of press releases should treat them as unverified.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Not licensed by the MHRA. Investigational only, available in the UK solely through participation in a registered clinical trial. Supply to the public would fall outside the Human Medicines Regulations 2012. |
| United States | Not approved. Investigational only. The phase 3 TRIUMPH programme has reported positive topline results by press release, and a regulatory submission to the FDA is stated by the sponsor as planned for the first quarter of 2027. Any retatrutide sold to consumers, including material labelled 'for research use only', is an unapproved drug being distributed outside the regulatory system. |
| WADA (sport) | Not specifically named on the WADA Prohibited List, but as a non-approved pharmacological substance it falls within class S0 (Non-Approved Substances) and is therefore prohibited at all times for athletes under the Code. |
Questions
No. As of August 2026 it is not approved by any regulator in any country. It exists legitimately only inside clinical trials. Everything sold under the name retatrutide on the consumer market is unapproved material of unverified identity and purity, and the 'for research use only' label such vendors use is a legal device that lets them sell an unapproved drug to the public, not a statement about quality.
The leading explanation is the glucagon receptor arm. Incretin agonism reduces how much you eat; glucagon receptor activation is thought to raise how much energy you burn and to drive hepatic fat oxidation. Working on both sides of energy balance plausibly explains the larger effect. That said, no head-to-head randomised trial against tirzepatide exists in any form, published or announced, so cross-trial comparisons of percentage weight loss are indicative rather than conclusive.
Phase 2 showed a dose-dependent rise in heart rate that peaked around week 24 and partly attenuated afterwards. Sustained heart rate elevation is a recognised cardiovascular concern in principle, and it is the sort of effect that only a large, long cardiovascular outcome trial can properly resolve. That trial has not been published. Until it is, the cardiovascular safety of retatrutide should be regarded as uncharacterised rather than reassuring.
Treat them cautiously. The 28.3% and 30.3% figures circulating widely come from a sponsor press release of 21 May 2026, not from a peer-reviewed paper or a posted results database entry. Press releases report favourable headline numbers without the full safety tables, discontinuation rates, subgroup analyses or independent statistical review that a published paper carries. Note also that the widely quoted figures are the best-performing dose and a selected BMI subgroup in an extension phase; the 4 mg arm reached 19.0%. They are worth knowing about; they are not the same category of evidence as the phase 2 papers.