Retatrutide research and evidence overview

The evidence for retatrutide consists of a small number of unusually strong published trials: a phase 2 obesity trial in NEJM reporting up to 24.2% mean weight loss at 48 weeks, a phase 2 type 2 diabetes trial in The Lancet, a liver-fat substudy in Nature Medicine, and — since 2026 — the first phase 3 results. What retatrutide does not have is any regulatory approval: it remains a clinical-stage compound, and everything sold under its name is unapproved gray-market product. Here is the record, trial by trial.

The trial that made retatrutide famous (Jastreboff et al., NEJM 2023) was a double-blind, randomized, placebo-controlled study of 338 adults with obesity or overweight, treated for 48 weeks with once-weekly retatrutide at maintenance doses of 1–12 mg:

• Mean weight change: −8.7% (1 mg) to −24.2% (12 mg), versus −2.1% for placebo.
• 60–83% of participants on active doses lost at least 15% of body weight, versus 2% on placebo.
• The 12 mg arm was −17.5% at the 24-week primary endpoint and −24.2% at 48 weeks, so weight was still falling steeply through the second half.
• GI adverse events were most common, dose-related, and mostly mild to moderate; heart rate increased, peaking at 24 weeks before declining.

No single-agent randomized trial had reported a mean reduction that large before. It is also, structurally, a 338-person dose-finding study — the kind of result phase 3 exists to confirm or temper.

A parallel phase 2 trial tested retatrutide in 281 people with type 2 diabetes over 36 weeks, against both placebo and dulaglutide 1.5 mg (Rosenstock et al., The Lancet 2023):

• HbA1c fell 2.02 percentage points at 24 weeks on 12 mg — superior to placebo and to dulaglutide.
• Body weight fell 16.94% at 36 weeks on 12 mg, versus 3.00% on placebo and 2.02% on dulaglutide.
• 35% of retatrutide participants had mild-to-moderate GI events; no severe hypoglycemia or deaths were reported.

The result mattered mechanistically: it showed the glucose-raising glucagon arm did not undermine glycemic control when packaged with GLP-1 and GIP agonism.

A phase 2a substudy enrolled 98 participants with metabolic dysfunction-associated steatotic liver disease (MASLD) and at least 10% liver fat (Sanyal et al., Nature Medicine 2024). At 24 weeks, mean relative liver-fat reductions were −42.9% (1 mg), −57.0% (4 mg), −81.4% (8 mg), and −82.4% (12 mg), versus +0.3% for placebo; 86% of the 12 mg group reached normal liver-fat levels (below 5%). This is the clearest published evidence for the design rationale of the glucagon arm — direct hepatic fat clearance beyond what appetite suppression alone would predict.

Eli Lilly's pivotal program spans obesity (TRIUMPH) and type 2 diabetes (TRANSCEND). Status on ClinicalTrials.gov as of August 2026:

• TRIUMPH-1 (NCT05929066) — obesity/overweight without diabetes, n=2,335, phase 3, completed.
• TRIUMPH-3 (NCT05882045) — severe obesity with established cardiovascular disease, n=1,946, ~113 weeks, completed.
• TRIUMPH-4 (NCT05931367) — obesity/overweight with knee osteoarthritis, completed.
• TRIUMPH-Outcomes (NCT06383390) — long-term cardiovascular and kidney outcomes, active, not recruiting.

The first phase 3 publication arrived in 2026: TRANSCEND-T2D-1 (n=537, 40 weeks) reported HbA1c reductions of 1.69–1.94 points and weight reductions of 11.5–15.3% across 4, 9, and 12 mg doses, with GI events again the most frequent adverse events and no severe hypoglycemia (Bajaj et al., The Lancet 2026).

Retatrutide is not approved by the FDA, the EMA, or any other regulator, for any indication. Legitimately, it exists only inside its clinical trials. Product sold by peptide vendors is unapproved material outside the regulated supply chain — a category the FDA has explicitly warned about for GLP-1 class drugs (FDA). For how retatrutide's record compares with its approved relatives, see tirzepatide, semaglutide, and the head-to-head comparison.

Three cautions keep the record in proportion. Phase 2 trials are dose-finding studies of a few hundred people — impressive effect sizes at that stage sometimes shrink in larger, longer trials. Cross-trial comparisons with semaglutide's and tirzepatide's pivotal programs are unreliable because populations, durations, and designs differ. And the questions that decide a drug's real-world standing — cardiovascular outcomes, rare harms, durability — are answered by the outcome trials still underway, not by the headline percentage. The published record is genuinely remarkable; it is also genuinely unfinished.