Retatrutide: What We Know About Lilly's Triple Agonist as TRIUMPH-4 Reads Out

Retatrutide (Eli Lilly internal designation LY3437943) is the most-anticipated obesity drug in the late-stage pipeline. It's a single molecule that simultaneously activates three different gut hormone receptors — GLP-1, GIP, and glucagon — making it the first triple agonist to reach phase 3 development. The phase 2 trial in adults with obesity (Jastreboff et al., NEJM 2023) reported up to 24.2% body weight reduction at 48 weeks on the highest dose [1] — the largest weight loss any pharmacological obesity therapy had ever produced in a controlled trial at the time of publication. The first phase 3 readout, TRIUMPH-4, came in on December 11, 2025 in adults with knee osteoarthritis and obesity: 28.7% mean weight loss (about 71 pounds) at the highest dose, plus 75.8% reduction in OA pain [3]. Seven additional TRIUMPH phase 3 readouts are expected throughout 2026. This article walks through the verified phase 2 and TRIUMPH-4 data, the proposed mechanism, and the new safety signal flagged in the phase 3 program.

What a triple agonist actually means

The body has multiple gut-hormone receptor systems that regulate appetite, glucose metabolism, and energy expenditure. The three relevant for retatrutide are:

• GLP-1 receptor: activated by semaglutide and tirzepatide. Reduces appetite, slows gastric emptying, increases insulin secretion in a glucose-dependent manner, and signals satiety in hindbrain reward circuits.
• GIP receptor: activated by tirzepatide (which is a dual GLP-1 + GIP agonist). Glucose-dependent insulinotropic polypeptide enhances insulin secretion and may modulate adipose tissue energy storage. The additive effect of GIP on top of GLP-1 is part of why tirzepatide outperforms semaglutide.
• Glucagon receptor: activated only by retatrutide in this drug class. Glucagon is best known as a hyperglycemic counter-regulatory hormone, but it also increases energy expenditure, stimulates lipolysis, and reduces hepatic fat. The glucagon-receptor arm is what makes retatrutide's effect size larger than tirzepatide's — but it also creates the most interesting safety questions, since you're simultaneously activating a hormone that raises blood glucose alongside two that lower it.

Coskun et al. published the discovery and preclinical proof-of-concept work for LY3437943 in Cell Metabolism in 2022 [5], establishing the receptor binding profile and the in vivo metabolic effects in animal models that justified progression to human trials.

The phase 2 trial: where the 24% number came from

Jastreboff et al., NEJM 2023, was the phase 2 randomized trial of retatrutide in adults with obesity (no diabetes) [1]. Verified design and result:

• Sample size: 338 adults randomized across multiple dose arms and placebo
• Doses: 1 mg, 4 mg (with two different escalation schedules), 8 mg, and 12 mg subcutaneously weekly
• Duration: 48 weeks
• Primary endpoint: percent change in body weight from baseline at week 24 (and week 48)

The 24.2% mean weight loss at the highest dose was unprecedented for the obesity drug class. For context, SURMOUNT-1 had reported tirzepatide 15 mg producing 20.9% at 72 weeks — a longer trial — and STEP-1 had reported semaglutide 2.4 mg producing 14.9% at 68 weeks. Retatrutide 12 mg was producing more weight loss in 48 weeks than either of the highest-effect-size drugs in the existing market did over their longer trial durations.

Jastreboff et al. also published the diabetes phase 2 trial in The Lancet in 2023 [6], showing dose-dependent weight loss and HbA1c reduction in T2D patients consistent with the obesity-trial signal.

The TRIUMPH phase 3 program

Lilly designed the TRIUMPH program as a portfolio of registrational trials across obesity, T2D, knee osteoarthritis, obstructive sleep apnea, cardiovascular outcomes, MASLD/MASH, and chronic low back pain. The rationale and design were published by Giblin et al. in Diabetes, Obesity and Metabolism in 2026 [2]. The full TRIUMPH program enrolls more than 5,800 participants across multiple trials.

The four headline trials are:

• TRIUMPH-1 (NCT05929066) [4]: phase 3 weight management in adults with obesity or overweight + ≥1 weight-related comorbidity (no T2D), with nested OSA and OA cohorts. Expected readout: 2026.
• TRIUMPH-2: phase 3 weight management basket trial including patients with T2D. Expected readout: 2026.
• TRIUMPH-3: phase 3 weight management in adults with established cardiovascular disease. Expected readout: 2026.
• TRIUMPH-4 (read out December 11, 2025): phase 3 stand-alone study in adults with obesity and knee osteoarthritis [3].

TRIUMPH-4: the first phase 3 readout

TRIUMPH-4 reported topline results on December 11, 2025 [3]. Verified details:

• Sample size: 445 participants
• Population: adults with obesity or overweight + knee osteoarthritis
• Duration: 68 weeks
• Doses tested: retatrutide 9 mg and 12 mg subcutaneously once weekly
• Primary endpoints: percent change in body weight, change in OA pain (WOMAC pain subscale)

Lilly reported that retatrutide met all primary and key secondary endpoints in TRIUMPH-4 [3]:

• Weight loss up to 28.7% (~71.2 lbs) at the highest dose
• OA pain reduction up to 75.8% from baseline

The 28.7% mean weight loss confirms — and meaningfully extends — the phase 2 signal. It is the largest weight loss any pharmacological therapy has ever shown in a randomized phase 3 trial. The OA pain reduction is also clinically transformative for the population enrolled in the trial; orthopedic guidelines have historically had very few non-surgical options that produce this magnitude of pain improvement in obese patients with knee OA.

The new safety signal

Lilly's December 2025 announcement and subsequent independent coverage flagged a new safety signal that emerged in the TRIUMPH-4 data. The exact nature and magnitude of the signal have not been fully characterized in publicly-available primary documents at the time of this writing, and the trial nonetheless met all primary and key secondary endpoints.

For YMYL editorial purposes, the responsible framing is that retatrutide is investigational, the phase 3 program is still in flight, and a complete safety profile will not be available until (a) the full TRIUMPH-4 manuscript is published, (b) the additional TRIUMPH readouts come in throughout 2026, and (c) the FDA review process produces a public risk-benefit assessment. Until then, any patient considering compounded retatrutide from a telehealth provider — and a small number of telehealth clinics have begun offering it — is doing so without the full FDA-reviewed safety dataset that the brand-name approved GLP-1s have accumulated. We'll update this article as the safety picture clarifies.

How retatrutide compares to the rest of the obesity drug landscape

Retatrutide is currently positioned to become the most effective obesity drug ever approved, assuming the TRIUMPH program continues to produce the kind of results TRIUMPH-4 reported and the safety signal does not derail the regulatory path. The relevant question for the next 12-18 months is not whether retatrutide will be approved but at what dose, with what label cautions, and at what price.

What about compounded retatrutide?

A small number of telehealth providers have begun offering compounded retatrutide directly to patients, framing it as a way to access the next-generation drug before FDA approval. We strongly recommend caution here for several reasons:

1. Retatrutide is not FDA-approved. It has not received any of the regulatory scrutiny that approved GLP-1s have. The full safety dataset is not public.
2. Compounding pharmacies cannot legally compound drugs that are still in active clinical investigation unless those drugs are on the FDA bulk substances list, which retatrutide is not. Compounded retatrutide is in a regulatory gray zone at best.
3. The active pharmaceutical ingredient (API) source is uncertain. Without an FDA-approved API supply chain, compounded retatrutide is being sourced from APIs that have not gone through the identity, purity, and potency verification that approved drugs require.
4. The dosing is not standardized. The phase 2 and phase 3 dose schedules used carefully characterized titration; off-label compounded use does not consistently follow these protocols.

For more on the FDA enforcement landscape around compounded GLP-1s, see our FDA warning letters investigation. For the difference between 503A and 503B compounding and what each can legally make, see our compounded semaglutide bioequivalence deep-dive.

Open questions

1. Full TRIUMPH-1, -2, -3 readouts. The headline obesity, T2D, and CVD trials are all expected in 2026. Magnitude, durability, and adverse event profile across these populations will determine the final approved indication and dose.
2. The glucagon-receptor safety question. Glucagon-receptor agonism is the novel mechanistic element. It also creates the most interesting safety questions: hepatic effects, glycemic control in T2D, cardiac effects of increased energy expenditure, and the still-unfolding safety signal flagged in TRIUMPH-4.
3. Pricing. Lilly has not announced launch pricing. Given the effect-size advantage, retatrutide is likely to launch at premium pricing relative to existing brand-name injectables.
4. Mortality and CV outcomes. The TRIUMPH cardiovascular outcomes trial will need years to mature. Until then, there is no hard-outcome retatrutide data analogous to the SELECT trial for semaglutide.

Related research

For the broader obesity drug landscape, see our Foundayo (oral orforglipron) approval analysis, the CagriSema REDEFINE trial deep-dive, and the tirzepatide vs semaglutide head-to-head. For the cardiovascular outcomes evidence on injectable semaglutide, see our SELECT trial deep-dive. For pricing context, see our GLP-1 pricing index.