Retatrutide Weight Loss: What Two Phase 3 Trials Found

Two Phase 3 trials of the investigational triple-agonist retatrutide found very large, but preliminary, average weight loss. Sleep and pain measures also improved.

In two Phase 3 trials, the investigational triple-agonist retatrutide produced average weight loss of about 25% at 80 weeks in adults with obesity but no diabetes, and about 19% in adults with obesity plus type 2 diabetes — the largest reported so far for an obesity drug candidate, but population-dependent, limited by side effects, and with no heart or mortality outcomes reported.

What this means

In two manufacturer-funded Phase 3 trials, the investigational drug retatrutide produced very large average weight loss at 80 weeks: about 25% at the 12 mg dose in adults with obesity but not diabetes, and about 19% in adults with obesity and type 2 diabetes. These are group averages, not what any individual should expect, and the trials differ in population, so their results are not directly comparable.

Gastrointestinal side effects, and in the diabetes trial some nerve-sensation and blood-pressure signals, were common and dose-related. Retatrutide was not yet approved, and neither trial reported whether it reduces heart attacks or death.

Two trials, one question

Retatrutide is an investigational triple agonist — one molecule built to switch on three hormone receptors (GIP, GLP-1 and glucagon). It is not approved for any use. Both trials tested it the same way: a once-weekly injection of 4, 9 or 12 mg, or a placebo injection, for 80 weeks. The comparator in each was placebo, not an existing obesity drug.

TRIUMPH-1, in the New England Journal of Medicine, enrolled 2,339 adults with obesity but not diabetes. TRIUMPH-2, in The Lancet, enrolled 1,152 adults with obesity and type 2 diabetes at 92 centres in eight countries. Because the two ran in different populations, lining their results up is informal: the gap between them may reflect who was studied as much as the drug.

How much weight loss — and in whom

In TRIUMPH-1, weight fell by an average of 25.0% at 12 mg, 23.7% at 9 mg and 17.6% at 4 mg after 80 weeks, against 3.9% on placebo. The difference from placebo was 21.0 percentage points at 12 mg and 19.8 points at 9 mg (both P<0.001). Two prespecified non-weight outcomes moved too: knee-arthritis pain scores fell more than on placebo, and sleep-apnea events per hour fell by roughly 21–24 an hour more than placebo in a 243-person subgroup.

In TRIUMPH-2, which studied people with diabetes, weight fell by an average of 18.8% at 12 mg, 16.8% at 9 mg and 11.9% at 4 mg, against 5.1% on placebo. The differences from placebo were 13.8 percentage points at 12 mg (95% CI −15.8 to −11.8), 11.8 points at 9 mg and 6.9 points at 4 mg, all with p<0.0001.

A 25% reduction is a proportion, not a fixed amount: for a person starting at 100 kg (about 220 lb), it would be roughly 25 kg (about 55 lb). That is arithmetic on a group average, and it does not predict any individual’s result.

The tolerability signal

Side effects were the main limit, and they rose with dose. In TRIUMPH-2, diarrhoea occurred in about 27% of the 4 mg group, 34% of the 9 mg group and 34% of the 12 mg group, against 13% on placebo; nausea rose with dose, reaching 28% at 12 mg against 8% on placebo. Two less usual signals stood out: unusual skin sensations (dysesthesia) in 4%, 6% and 7% across the doses against 1% on placebo, and low blood pressure (hypotension) in 1%, 5% and 6% against under 1% on placebo. Twelve per cent of the 9 mg group stopped treatment permanently because of side effects or death, against 5% on placebo; 84% completed the study drug. Seven deaths occurred across the trial’s groups (six on retatrutide, one on placebo), all judged by the investigators to be unrelated to the drug. TRIUMPH-1 reported that its most common adverse events were gastrointestinal and dose-related.

What the trials can’t tell you

Three limits shape how far these numbers reach. First, the abstracts are all we could read: confidence intervals for the TRIUMPH-1 differences, that trial’s discontinuation figures, subgroup detail and individual conflict statements were unavailable to us.

Second, neither trial reported cardiac or mortality outcomes. Weight loss of this size is a surrogate measure; the trials were not designed or powered to show whether retatrutide reduces heart attacks, strokes or deaths. How much lean mass is lost alongside fat is a related class question these trials do not resolve here.

Third, both trials were funded and run by the drug’s maker, Eli Lilly. In TRIUMPH-1 several authors are Lilly employees, though the individual conflict statement is paywalled. In TRIUMPH-2, six authors are Lilly employees and shareholders, and lead investigators report wide industry ties to companies including Eli Lilly, Novo Nordisk and Sanofi. Manufacturer funding does not make a result false, but it is a reason to read a single-company evidence base with caution; this packet has no independent replication, and retatrutide has not been compared head-to-head against existing obesity drugs.

Where this sits

These trials confirm and extend an already-strong signal. A 2023 Phase 2 trial (n=338, 48 weeks) found 24.2% average weight loss at 12 mg, establishing the dose-response the Phase 3 trials now carry to 80 weeks in two populations. What the pair adds is scale, duration and the plain finding that the same drug produces smaller losses in people with type 2 diabetes. What remains open is the class-level worry that very large weight loss can carry away lean mass along with fat — the subject of a 2026 review of the sarcopenia concern with incretin-based therapies — and whether any of this translates into fewer heart attacks or deaths. Retatrutide remains investigational; nothing here is a reason to start, switch or stop any medication.

Related coverage: why the same GLP-1 drugs work differently for different people (Why do GLP-1 weight-loss drugs work differently for different people?), and what happens when people stop weight-loss medications (What Happens When People Stop Weight-Loss Medications).

Terms explained

Triple hormone receptor agonist: a drug built to activate three hormone receptors at once. Retatrutide targets GIP, GLP-1 and glucagon, the gut and metabolic hormones that shape appetite, digestion and blood-sugar handling.
Phase 3 trial: the large, late-stage human trial that tests whether a treatment works and what its side effects are, usually against a placebo or standard treatment. It is the stage before regulators decide whether to approve a drug.
Treatment-regimen estimand: an analysis approach that counts every participant in the group they were assigned to, rather than only those who completed treatment — generally a more conservative, real-world-leaning picture than an analysis of completers alone.
Dysesthesia: an abnormal, often unpleasant skin sensation — tingling, burning or prickling — without an obvious external cause. It was one of the less common side effects in the diabetes trial.

Sources

Primary source (TRIUMPH-1): Jastreboff AM, Kaplan LM, Davies MJ, Wilding JPH, Ahmad NN, Murray M, Du Y, Makarova N, Giblin K, Schloot NC, Wang Y. “Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity.” N Engl J Med 2026, published online 2026-09-29 (ahead of print). DOI: 10.1056/NEJMoa2604169 · PMID: 42814954 · registered as NCT05929066. Peer-reviewed Phase 3 randomised, double-blind, placebo-controlled trial, n=2,339, 80 weeks; funded by Eli Lilly. Abstract-level access (the full paper was not available to us).
Companion source (TRIUMPH-2): Bellido V, le Roux CW, Ekinci EI, Bjornstad P, Frias JP, Giblin K, Eyde S, Park SY, Ferro J, Johnson C, Taylor A. “Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2): a double-blind, parallel-group, randomised, placebo-controlled, phase 3 trial.” Lancet 2026, published online 2026-09-29. DOI: 10.1016/S0140-6736(26)01861-101861-1) · PMID: 42810372 · registered as NCT05929079. Peer-reviewed Phase 3 trial at 92 centres in eight countries, n=1,152, 80 weeks; funded by Eli Lilly, with six authors employed by and holding shares in the company. Abstract-level access (the full paper was not available to us).
Landscape — Phase 2 predecessor: Jastreboff AM, et al. “Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial.” N Engl J Med 2023;389(6):514-526. DOI: 10.1056/NEJMoa2301972 · PMID: 37366315 (abstract-level access; n=338, 48 weeks, −24.2% at 12 mg).
Landscape — lean-mass concern: “Beyond Fat Loss: Addressing the Sarcopenia Challenge of Incretin-Based Therapies.” Diabetes Metab J 2026;50(5):844-862. DOI: 10.4093/dmj.2026.0702 · PMID: 42722378 (abstract-level access; review of the lean-mass/sarcopenia question with incretin-based therapies).