Peptide Science

Retatrutide Results: What Phase 2 Showed

By UK Peptide Lab•26 July 2026•9 min read

Key Takeaways

The phase 2 obesity trial randomised 338 adults across placebo and several retatrutide dose arms over 48 weeks.
Reported mean weight change at 48 weeks rose across the dose arms, reaching roughly a quarter of body weight in the highest.
A separate phase 2 trial in type 2 diabetes reported reductions in both HbA1c and body weight against placebo and an active comparator.
Trial arms included different escalation routes to the same target dose, which is why escalation speed can be separated from dose in the results.
These outcomes were produced under medical supervision with monitoring, defined escalation and the ability to hold or reduce exposure.
Phase 3 (TRIUMPH) has not reported. No regulator anywhere has approved retatrutide.

Why the design matters more than the headline

The retatrutide weight-change figure gets quoted more than almost any number in metabolic research, and almost always stripped of the trial that produced it. That matters, because the design determines what the number means. A mean weight change across a dose arm is not a result an individual can expect. A 48-week outcome says nothing about year three. And an outcome produced under supervision, with structured escalation and the ability to hold or reduce exposure, is not the same phenomenon as the same molecule used without any of that. What follows is what the trials tested and what they reported, with the conditions attached.

The phase 2 obesity trial

Jastreboff and colleagues reported the trial in the New England Journal of Medicine in 2023. It randomised 338 adults with obesity, or with overweight plus at least one weight-related condition, to placebo or one of several retatrutide dose arms, administered by weekly subcutaneous injection over 48 weeks. The design included a feature worth understanding. Rather than testing one escalation path per dose, it included arms reaching the same target through different escalation routes, one faster and one slower. That is what allows dose and rate of change to be examined separately, and it is why the trial can say something about tolerability that a simpler design could not. The reported mean weight change increased across the dose arms over the 48 weeks, reaching approximately a quarter of body weight in the highest arm, against a small change in the placebo group. The full arm-by-arm figures are in the published paper, linked in the references below, and are worth reading at source rather than through a summary.

The phase 2 type 2 diabetes trial

Rosenstock and colleagues reported a separate phase 2 trial in the Lancet in 2023, in people with type 2 diabetes, comparing retatrutide against both placebo and an active comparator from the GLP-1 receptor agonist class. This trial reported reductions in HbA1c alongside reductions in body weight. Its inclusion of an active comparator makes it the more informative of the two for placing retatrutide against existing therapy, though the comparator was a GLP-1 agonist rather than the dual agonist most people actually want it compared with. The two trials answer different questions and should not be pooled. Different populations, different endpoints, different durations.

What the numbers do not tell you

Four things, all of which matter and none of which appear in the headline figure. Means conceal spread. A mean weight change across an arm is compatible with a wide distribution of individual outcomes within it, including participants who changed very little. Reporting the mean as though it were a typical individual result misrepresents the data. Trials carry surrounding care. Participants receive monitoring, structured escalation, lifestyle guidance and clinical contact throughout. That support is part of what produced the result, and it does not travel with the molecule. Completers are not everybody. People who discontinue, including for adverse events, leave the trial. Outcome figures are shaped by who remained, and discontinuation was concentrated in the higher-dose arms. Forty-eight weeks is not durable. What happens over several years, including on cessation, is unstudied for this compound.

The mechanism behind the outcomes

Retatrutide is a synthetic single-chain peptide acting as an agonist at three receptors simultaneously: GIP, GLP-1 and glucagon. Coskun and colleagues (Cell Metab, 2022) reported the discovery pharmacology and characterised the in vitro balance, describing greater potency at GIPR relative to GLP-1R and GCGR rather than equal engagement of all three. The glucagon arm is the part that distinguishes it. Glucagon receptor activation raises hepatic energy expenditure and promotes lipolysis, but on its own it also raises circulating glucose. Pairing it with two incretin arms is intended to let the thermogenic effect proceed while the glycaemic consequence is counterbalanced. Whether that mechanism explains the magnitude of the reported outcomes, or whether it is largely the incretin arms doing the work at higher exposure, is not settled by the phase 2 data.

Where the programme stands

Retatrutide is in phase 3. The TRIUMPH programme is running across several indications and has not reported. Until it does, everything above remains phase 2 evidence: informative about direction and magnitude, underpowered for uncommon adverse events, and not the basis on which any regulator approves a medicine. Retatrutide holds no marketing authorisation in the UK, the EU, the US or anywhere else, and no licensed formulation of it exists. For the adverse events reported alongside these outcomes, see our summary of what the trials reported about retatrutide side effects. Retatrutide supplied by UK Peptide Lab is a lyophilised research chemical for in-vitro laboratory use only, not for human consumption, veterinary use, or any therapeutic purpose.

Related Products

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Disclaimer: This article is for research and educational purposes only. All information provided is not intended as medical advice. UK Peptide Lab products are not for human consumption and are sold strictly for laboratory research use only.

Frequently Asked Questions

What were the results of the retatrutide phase 2 trial?

The obesity trial reported by Jastreboff and colleagues (NEJM, 2023) randomised 338 adults across placebo and several retatrutide dose arms and ran for 48 weeks. Mean weight change increased across the dose arms, reaching approximately a quarter of body weight in the highest-dose arm at 48 weeks, against a small change on placebo. A separate phase 2 trial in type 2 diabetes reported reductions in both HbA1c and body weight against placebo and an active comparator. Both are phase 2 results in supervised trial conditions.

What doses were used in the retatrutide trials?

The phase 2 obesity trial tested several weekly subcutaneous dose levels alongside placebo, with the highest arm at 12mg, and it deliberately included more than one escalation route to the same target so that the effect of escalation speed could be separated from the effect of dose. This is a description of how the trial was designed, published in the New England Journal of Medicine. It is not a protocol, and nothing on this site tells anyone what quantity to use.

How much weight was lost in the retatrutide trial?

Mean weight change rose across the dose arms over the 48-week period, reaching roughly a quarter of body weight in the highest-dose arm, with placebo showing only a small change. Two caveats matter when reading that. It is a mean, so individual results within each arm varied considerably around it. And it was produced inside a clinical trial with medical supervision, structured escalation and the surrounding care those trials provide.

Is retatrutide better than tirzepatide or semaglutide?

No head-to-head trial has ever been run, so this cannot be answered from evidence. The figures being compared come from separate trials with different populations, durations, escalation schedules and endpoints, and placing them side by side produces a comparison that looks rigorous and is not. What is true is that retatrutide adds glucagon receptor agonism to the incretin mechanisms the other two use, which is a genuine pharmacological difference. Whether that translates into superiority in a controlled comparison is unknown.

Has retatrutide finished clinical trials?

No. Retatrutide completed phase 2 and is in phase 3, the TRIUMPH programme, which has not reported. It holds no marketing authorisation in the UK, the EU, the US or anywhere else, and there is no licensed formulation of it in the world. It is supplied here strictly as a research chemical for in-vitro laboratory use.

References

[1] Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial (Jastreboff et al.). New England Journal of Medicine (2023). View →
[2] Retatrutide (LY3437943) in people with type 2 diabetes: a randomised, double-blind, placebo- and active-controlled phase 2 trial (Rosenstock et al.). The Lancet (2023). View →
[3] LY3437943, a novel triple glucagon, GIP and GLP-1 receptor agonist: from discovery to clinical trials (Coskun et al.). Cell Metabolism (2022). View →