Retatrutide has passed every Phase 3 test so far. You still can’t legally buy it.
Five pivotal trials, weight loss in bariatric-surgery territory, and a filing date. Meanwhile, the vials sold online are an entirely different product.
What it is
Retatrutide (LY3437943) is Eli Lilly’s once-weekly injectable that acts on three hormone receptors at once: GIP, GLP-1 and glucagon. Semaglutide acts on one of these receptors and tirzepatide on two. The third, glucagon, is the new element, and it is the reason researchers expect effects on energy expenditure and liver fat that the older drugs don’t produce.
Where it started
Phase 2 is where the “biggest number ever” reputation began. In the obesity trial, published in the New England Journal of Medicine in 2023, adults with obesity lost an average of 24.2% of body weight at 48 weeks on the highest dose, against 2.1% on placebo, and the investigators noted that weight had not yet plateaued when the study ended.
A companion trial in The Lancet tested it in 281 adults with type 2 diabetes against both placebo and dulaglutide. At the top dose, HbA1c fell by about 2.0 percentage points at 24 weeks, a larger drop than dulaglutide produced, and the results held at 36 weeks. A substudy of the obesity trial found liver fat fell by 82.4% at 24 weeks on the top dose, with 86% of those participants reaching normal liver fat levels. That result is why fatty liver disease is now one of the Phase 3 indications.
What the Phase 3 trials show
The main obesity trial, TRIUMPH-1, randomised 2,339 adults with obesity or overweight and no diabetes to one of three doses or placebo for 80 weeks. On the highest dose, participants lost an average of 28.3% of body weight, against 2.2% on placebo, and 45.3% of them lost at least 30%. In an extension for people starting with a BMI of 35 or more, average loss reached 30.3% at 104 weeks. Even the lowest dose, reached with a single escalation step, produced 19.0% average loss.
Harder populations saw smaller but still large effects. In TRIUMPH-2, which enrolled 1,152 adults with type 2 diabetes, weight loss ranged from 12.7% to 20.8% across the doses versus 4.0% on placebo, with HbA1c falling by up to 1.6 points. TRIUMPH-3, in adults with severe obesity and established cardiovascular disease, reported losses of up to 22.6% at 80 weeks. TRIUMPH-4, in people with knee osteoarthritis, reported 28.7% average loss at 68 weeks.
What it costs you
The side effects follow the same pattern as the rest of the class, at higher rates. On the top dose, 42.4% of participants reported nausea (14.8% on placebo) and 25.3% reported vomiting (4.8% on placebo). One side effect is new to this class: dysesthesia, an abnormal skin sensation, occurred in about 12% of participants on the two higher doses versus 0.9% on placebo, though most cases were mild and resolved. On the highest dose, 11.3% of participants stopped treatment because of side effects, compared with 4.9% on placebo.
Where it stands legally
It is not approved anywhere. Lilly states that retatrutide is legally available only to participants in its clinical trials. The company plans to file a Biologics License Application with the FDA in Q1 2027, which puts a US decision in late 2027 at the earliest.
US federal law bars pharmacies from compounding it, and the FDA is actively pursuing sellers. A March 2026 warning letter classified “research” retatrutide sold online as an unapproved new drug, and that April the agency sent seven warning letters to sellers who labelled peptides “research use only” while marketing appetite and fat-loss effects.
What’s in the vial
The trial data comes from Lilly’s product. The retatrutide sold online is not that product. An analysis of clinical records found use of non-trial products claiming to contain retatrutide growing 1.8-fold per quarter from late 2023 to March 2026. An independent obesity specialist said the data signals that people are buying it through channels where dose, purity, manufacturing quality and safety cannot be verified.
The analysis reported less weight loss and more cardiovascular effects among these users than in the trials. That is one study with acknowledged limitations, but it is the only real-world evidence available.
What nobody knows yet
Full peer-reviewed results for TRIUMPH-1, -2 and -3 are still pending; everything above comes from company topline releases. There is no head-to-head trial against tirzepatide. The cardiovascular outcomes trial has not reported, and nothing is known about what happens after two years of treatment or after stopping.
Evidence grade: Human trials
The randomised data is large and consistent. It covers only the manufacturer’s product, and it does not extend to anything sold as a research chemical.
Sources: Jastreboff et al., N Engl J Med 2023 (PubMed 37366315); Rosenstock et al., Lancet 2023 (PubMed 37385280); Eli Lilly topline releases, 21 May and 23 July 2026; ClinicalTrials.gov NCT05929066; FDA warning letter 721806 (March 2026); Medscape, gray-market retatrutide analysis (2026).
Disclosure: Peptide Curious’s publishers work commercially in this field. One of them operates a telehealth practice that intends to prescribe retatrutide if it is approved. Nothing in this dispatch links to any product or service. See our disclosure. Nothing here is medical advice, and nothing here is a recommendation to use any compound.
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