Retatrutide Clinical Trials: What the Data Actually Shows

Trial program summary

Retatrutide's development program is unusually broad for a drug at this stage. Most GLP-1 drugs run a single pivotal weight-loss trial first, then add diabetes and cardiovascular trials after approval. Lilly is running all four arms in parallel, which signals high confidence in the molecule and a strategic intent to launch with the broadest possible label.

Phase 2: the trial that started everything

Citation:Jastreboff AM et al., “Triple–Hormone-Receptor Agonist Retatrutide for Obesity,” New England Journal of Medicine, August 2023.

338 adults with obesity (BMI ≥30, or ≥27 with at least one weight-related comorbidity) were randomized to placebo or one of four retatrutide weekly doses for 48 weeks. The primary endpoint was percent change in body weight from baseline.

Headline results

• 12 mg weekly: 24.2% mean body weight loss at 48 weeks
• 8 mg weekly: 22.8% mean body weight loss
• 4 mg weekly: 17.1% mean body weight loss
• 1 mg weekly: 8.7% mean body weight loss
• Placebo: 2.1% mean body weight loss

What the curve showed

The most striking detail wasn't the magnitude — it was the trajectory. At 48 weeks, weight loss in the 8 mg and 12 mg arms was still actively dropping, with no sign of the plateau that other GLP-1 trials have shown by week 36 to 40. This is why Phase 3 is running for 80 weeks: to find out where the curve actually flattens.

Liver fat reduction

A pre-specified MRI-based subgroup analysis showed retatrutide produced larger reductions in liver fat than any GLP-1 medication previously studied at the same time point. This finding directly motivated the TRIUMPH-4 MASH program and is the basis for the “next-generation liver drug” framing that has appeared in industry coverage.

Side effect profile

Adverse events were predominantly gastrointestinal — nausea, diarrhea, vomiting, constipation — consistent with the GLP-1 class. Most were mild to moderate, dose-dependent, and concentrated in the first 12 weeks of titration. Two notable signals tied to the glucagon-receptor activation:

• Heart rate: mean increase of approximately 6 beats per minute at the 12 mg dose, plateauing within the first 12 weeks
• Glucose: small increases in fasting glucose in some non-diabetic participants, again attributed to the glucagon component, generally returning toward baseline with continued treatment

TRIUMPH-1: the obesity pivotal

ClinicalTrials.gov ID

NCT05882045

Population

Adults with obesity (BMI ≥30, or ≥27 with weight-related comorbidity), no diabetes

Primary endpoint

Percent change in body weight at 80 weeks

Status

Enrollment complete; readout expected 2026

This is the trial that determines whether retatrutide gets approved for weight management. The 80-week duration is longer than tirzepatide's 72-week SURMOUNT-1 trial, intentionally designed to fully characterize the weight-loss curve's plateau point. If the magnitude holds — somewhere between 22% and 28% mean weight loss — retatrutide will set a new bar for the class.

TRIUMPH-2: obesity with type 2 diabetes

ClinicalTrials.gov ID

NCT05929066

Population

Adults with obesity and type 2 diabetes

Primary endpoint

Body weight reduction and HbA1c reduction at 80 weeks

Status

Enrollment complete; readout expected 2026

This trial determines whether retatrutide gets a separate label for type 2 diabetes — the same dual-indication playbook Lilly used with tirzepatide (Mounjaro for diabetes, Zepbound for weight). The glucagon-receptor activation is the wild card here. Glucagon raises blood sugar, but the offsetting GLP-1 and GIP effects should net out to better glycemic control. Phase 2 supported this; Phase 3 will confirm in a larger diabetic population.

TRIUMPH-3: cardiovascular outcomes

ClinicalTrials.gov ID

NCT05882110

Population

Adults with obesity and established cardiovascular disease

Primary endpoint

Composite of cardiovascular death, non-fatal MI, non-fatal stroke (MACE)

Status

Long-running outcomes trial; readout expected 2027 or later

This is event-driven, not calendar-driven. The trial reads out when enough cardiovascular events (heart attacks, strokes, cardiovascular deaths) have accumulated across both treatment and placebo arms to power a meaningful statistical comparison. Semaglutide's SELECT trial took roughly 4 years from first enrollment to publication. Expect TRIUMPH-3 on a similar arc. A positive result would unlock a cardiovascular risk-reduction label, the same one Wegovy got in 2024.

TRIUMPH-4: the MASH program

ClinicalTrials.gov ID

NCT05989711

Population

Adults with obesity and biopsy-confirmed MASH

Primary endpoint

MASH resolution without worsening of fibrosis, and fibrosis improvement without worsening of MASH (on follow-up biopsy)

Status

Active; biopsy follow-up timeline

MASH (formerly NASH) is fatty liver disease with active inflammation. It progresses to cirrhosis in a subset of patients and is becoming the leading reason for liver transplant in the U.S. As of early 2026, only one drug (resmetirom, brand name Rezdiffra) is FDA-approved specifically for MASH.

Retatrutide's Phase 2 liver-fat data was the strongest of any GLP-1 to date. If TRIUMPH-4 confirms histological improvement on biopsy, retatrutide could be the first GLP-1 with a specific FDA-approved MASH indication — a major label differentiator vs Wegovy and Zepbound, both of which improve liver fat measurably but lack a dedicated MASH approval.

What each readout will tell us

• If TRIUMPH-1 confirms 22%+ weight loss: retatrutide moves to FDA submission and approval becomes the base case. Lilly likely files in 2026 with a target launch in 2027.
• If TRIUMPH-1 disappoints (under 18%): Lilly may delay submission to gather longer-term data, or seek a narrower initial label. Approval could push to 2028 or later.
• If TRIUMPH-2 succeeds: retatrutide gets a separate brand and label for type 2 diabetes, mirroring the Mounjaro/Zepbound split. This is a meaningful revenue and access expansion.
• If TRIUMPH-3 succeeds: retatrutide gets a cardiovascular risk-reduction label, which can unlock broader insurance coverage (insurers cover drugs with prevention claims more readily than weight-only drugs).
• If TRIUMPH-4 succeeds: retatrutide becomes the first GLP-1 with a dedicated MASH indication, a market currently dominated only by resmetirom (Rezdiffra). This is the clearest differentiator vs Wegovy and Zepbound.

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