Weight & Metabolic
Retatrutide
aka LY3437943 · 'triple-G' agonist · reta · retta · triple g · triple agonist · glp-3
Grade
An experimental once-weekly injectable that hits three metabolic hormone receptors at once and has produced the largest trial weight-loss results seen so far. It is not yet an approved medicine.
- Class
- Synthetic triple receptor agonist peptide (GLP-1 / GIP / glucagon)
- Evidence
- Grade B · Promising human evidence
- Sport / WADA
- Not a specific WADA-listed substance, but metabolic modulators and unapproved agents are scrutinised in sport; athletes should treat any unapproved peptide with caution.
- Last reviewed
- 2026-06
Grade B · Promising human evidence
Why this grade
A genuine investigational medicine with large, published, randomised human trials showing some of the biggest weight-loss figures recorded. It scores B rather than A only because it is still in Phase 3 and not yet approved. The human evidence is strong and growing, but not yet final.
What is it?
Retatrutide is a new weight-loss injection being tested by a big drug company. It copies three of the body's own 'I'm full / handle sugar / burn fuel' signals at the same time, instead of just one like the current injections. In studies, people lost more weight than with any existing medicine. It works in proper human trials, but it hasn't finished testing yet, so you can't get it prescribed.
If the current weight-loss injections are a car with one strong engine (GLP-1), retatrutide is a car with three engines wired together, and in testing it's gone noticeably further on the same journey. Unlike most peptides people discuss, this car has actually been driven on real roads with real drivers in proper trials. It just hasn't passed its final road-legality test yet.
How is it meant to work?
Retatrutide is a unified peptide that switches on three receptors at once. GLP-1 and GIP agonism reduce appetite and improve blood-sugar handling. Glucagon-receptor agonism is thought to increase energy expenditure and mobilise fat from the liver. The combination drives weight loss beyond what single- or dual-agonists achieve.
What's it studied for?
Research contexts. Not proven uses, and not recommendations.
Does the human evidence stack up?
Strong and published. The Phase 2 obesity trial in the New England Journal of Medicine reported up to ~24% body-weight reduction at 48 weeks (the highest seen for an anti-obesity agent at the time), with parallel diabetes and fatty-liver data. The Phase 3 TRIUMPH programme is in progress and will decide durability, cardiovascular safety and approval. This is real clinical-grade evidence, not extrapolation.
What could go wrong?
- !Not yet approved or licensed anywhere. It remains investigational, so safety conclusions are provisional until Phase 3 completes.
- !Dose-related gastrointestinal side effects (nausea, vomiting, diarrhoea) are common, as with the whole incretin class.
- !Transient increases in heart rate have been reported and are still being characterised.
- !Because it is unapproved, any 'retatrutide' sold online is grey-market, unregulated and of unverified identity and purity. This is a very different and much riskier proposition than a future licensed product.
- !Long-term outcomes, muscle-mass effects and weight regain after stopping are not yet established.
Is it legal in the UK?
Retatrutide is an investigational medicine with no MHRA marketing authorisation. It is not available on prescription or legally for sale in the UK. It can only be received as a participant in an approved clinical trial. Any product marketed to UK consumers as 'retatrutide' is unlicensed and unlawful to supply for human use. For contrast, related GLP-1 medicines such as semaglutide and tirzepatide are MHRA-approved and prescription-only.
Key trials
- NCT04881760· Phase 2· Completed / published
Phase 2 dose-finding study in obesity (Jastreboff 2023)
Basis for the headline ~24% weight-loss figure.
- TRIUMPH programme· Phase 3· Ongoing
Phase 3 obesity & related outcome trials
Will determine durability, safety and whether retatrutide is approved.
Sources
- 01Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — Jastreboff AM et al., New England Journal of Medicine (2023)
The landmark Phase 2 obesity RCT reporting up to ~24% weight loss at 48 weeks.
- 02Retatrutide for type 2 diabetes: a randomised, double-blind, Phase 2 trial — Rosenstock J et al., The Lancet (2023)
Companion Phase 2 trial in type 2 diabetes.
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