Why the trial results drew attention
Retatrutide is a triple agonist — it activates the GLP-1, GIP, and glucagon receptors together. The glucagon receptor is associated in research with energy expenditure, and it is this third pathway that researchers believe sets retatrutide apart from single-target GLP-1 compounds. In clinical trials, that combination has been associated with weight reductions larger than those reported for earlier agents.
Everything below is a summary of published clinical trial data in human study populations. It is provided for research and educational context and is not a claim about, or an intended use of, any product sold on this site.
The key clinical trials
Phase 2 · NEJM 2023
The trial that put retatrutide on the map
Jastreboff and colleagues reported a 338-participant, randomised, double-blind, placebo-controlled Phase 2 trial in the New England Journal of Medicine (2023). Adults with obesity (or overweight with a weight-related condition) received once-weekly retatrutide or placebo.
Phase 3 · TRIUMPH-1 2026
Confirmation at pivotal scale
The pivotal Phase 3 TRIUMPH-1 trial enrolled 2,339 adults with obesity or overweight and at least one weight-related condition (without type 2 diabetes). Topline results were reported by the manufacturer in 2026.
In a TRIUMPH-1 extension, participants with a higher baseline BMI continued to lose weight, reaching an average of around 30.3% at 104 weeks. Reductions of this magnitude approach those historically associated with bariatric surgery, which is why the results have been so widely discussed.
How the three pathways relate to body weight
Associated with appetite and satiety signalling and the rate of gastric emptying.
Studied for its roles in insulin secretion and in how the body handles lipids and stores energy.
Associated with energy expenditure and hepatic glucose and fat handling — the arm thought to amplify the effect seen in trials.
Regulatory status — read this first
As of 2026, retatrutide is an investigational compound. It is not approved or licensed as a medicine in the UK, the US, or anywhere else, and it cannot be prescribed or sold as a weight-loss treatment. A regulatory submission is anticipated, but that has not yet happened. RETApro supplies retatrutide strictly for laboratory and research use only — not for human or animal use, and not to diagnose, treat, cure, or prevent any disease.
Sources
- Jastreboff AL, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine, 2023.
- Eli Lilly and Company. Phase 3 TRIUMPH-1 topline results. Company announcement, 2026.
Frequently asked questions
How much weight loss did retatrutide produce in clinical trials?
In the Phase 2 trial published in the New England Journal of Medicine (2023), participants on the 12 mg dose lost an average of 24.2% of their body weight over 48 weeks, compared with 2.1% on placebo. In the larger Phase 3 TRIUMPH-1 trial reported in 2026, the 12 mg dose produced a mean reduction of 28.3% at 80 weeks under the efficacy estimand (25% under the more conservative treatment-regimen estimand), versus roughly 2–4% on placebo. These are trial findings in clinical populations, not outcomes we claim for any product.
Why is retatrutide studied for weight loss specifically?
Retatrutide engages three receptors — GLP-1, GIP, and glucagon — at once. Researchers are interested in the glucagon arm in particular because it is associated with energy expenditure, which distinguishes retatrutide from single-target GLP-1 compounds and is one reason the reductions seen in trials have been unusually large.
Is retatrutide an approved weight-loss medicine?
No. As of 2026 retatrutide is an investigational compound and is not approved or licensed as a medicine anywhere. A regulatory submission is anticipated but it cannot be prescribed or sold as a treatment. RETApro supplies retatrutide strictly for laboratory and research use only — not for human or animal use.
What side effects were seen in the trials?
The most frequently reported adverse events in the trials were gastrointestinal — nausea, diarrhoea, vomiting, and constipation. They were dose-related and mostly mild to moderate, and were reduced by lower starting doses with gradual escalation. This is reported here as trial data for research context only.
Research-grade retatrutide, independently tested
If you are sourcing retatrutide for laboratory research, every RETApro batch ships with an independently verifiable Certificate of Analysis and cold-chain packaging.