How much weight loss does retatrutide cause in trials?

Phase 2 Obesity Trial

The 2023 New England Journal of Medicine Phase 2 obesity trial randomized 338 adults to weekly retatrutide or placebo for 48 weeks. Least-squares mean body-weight change at 48 weeks was:

• Combined 4 mg group: 17.1%
• Combined 8 mg group: 22.8%
• 12 mg group: 24.2%
• Placebo: 2.1%

At least 15% weight loss occurred in 60% of participants in the 4 mg group, 75% in the 8 mg group, and 83% in the 12 mg group. This is the source of the 24.2% figure.

TRIUMPH-1 Phase 3

TRIUMPH-1 (NCT05929066) is Lilly's Phase 3 obesity or overweight trial in adults without type 2 diabetes. Lilly's May 21, 2026 topline release reported these body-weight changes at 80 weeks using the efficacy estimand:

• Retatrutide 4 mg: 19.0%
• Retatrutide 9 mg: 25.9%
• Retatrutide 12 mg: 28.3%
• Placebo: 2.2%

Using the treatment-regimen estimand at 80 weeks, Lilly reported 17.6% with 4 mg, 23.7% with 9 mg, 25.0% with 12 mg, and 3.9% with placebo. The estimand matters: the efficacy estimand estimates results had participants remained on study intervention without prohibited weight-management treatments; the treatment-regimen estimand estimates average effect regardless of adherence or prohibited weight-management treatments.

TRIUMPH-1 also included a pre-specified 104-week extension in participants with baseline BMI ≥35. Lilly reported 30.3% weight loss for the 12 mg-to-maximum-tolerated-dose arm using the efficacy estimand. The 28.7% TRIUMPH-1 extension number appears in the treatment-regimen estimand for the 9 mg-to-maximum-tolerated-dose arm at 104 weeks, not in the 80-week TRIUMPH-1 primary endpoint.

Other Phase 3 Readouts

• TRIUMPH-4 (NCT05931367): adults with obesity or overweight and knee osteoarthritis, without diabetes. Lilly reported 28.7% weight loss with 12 mg and 26.4% with 9 mg at 68 weeks using the efficacy estimand; treatment-regimen results were 23.7% with 12 mg and 20.0% with 9 mg.
• TRIUMPH-2 (NCT05929079): adults with obesity or overweight and type 2 diabetes. Lilly reported 20.8% weight loss with 12 mg, 19.1% with 9 mg, and 12.7% with 4 mg at 80 weeks using the efficacy estimand.
• TRIUMPH-3 (NCT05882045): adults with severe obesity and established cardiovascular disease. Lilly reported 22.6% weight loss with 12 mg and 21.6% with 9 mg at 80 weeks using the efficacy estimand.

Results in Context

• Semaglutide 2.4 mg (STEP 1, 68 weeks): 14.9% mean weight loss
• Tirzepatide 15 mg (SURMOUNT-1, 72 weeks): 22.5% mean weight loss
• Retatrutide 12 mg (Phase 2 obesity trial, 48 weeks): 24.2% least-squares mean weight loss
• Retatrutide 12 mg (TRIUMPH-1 Phase 3, 80 weeks): 28.3% efficacy-estimand weight loss

These are cross-trial comparisons, not head-to-head trials. Population, follow-up duration, dose escalation, adherence assumptions, and estimand all affect the number.

What the Main Numbers Mean

• 24.2%: Phase 2 obesity trial, 12 mg, 48 weeks, least-squares mean body-weight change
• 28.3%: TRIUMPH-1 Phase 3 obesity trial, 12 mg, 80 weeks, efficacy estimand
• 28.7%: TRIUMPH-4 Phase 3 knee osteoarthritis trial, 12 mg, 68 weeks, efficacy estimand; also appears in TRIUMPH-1 extension treatment-regimen results for the 9 mg-to-maximum-tolerated-dose arm at 104 weeks

Why Retatrutide Produces More Weight Loss

Triple receptor agonism

• GLP-1 receptor: appetite suppression, delayed gastric emptying, insulin sensitization
• GIP receptor: additional appetite effects, metabolic sensitization
• Glucagon receptor: increased hepatic lipolysis, energy expenditure

The glucagon receptor activation is the key additional mechanism not present in semaglutide (GLP-1 only) or tirzepatide (GLP-1/GIP). It increases baseline energy expenditure and promotes fat utilization — adding a "burn" component to the "eat less" effect of GLP-1 class drugs.

Comparison to Bariatric Surgery

• Gastric bypass: 25–35% total weight loss, 12–24 months
• Retatrutide 12 mg: 28.3% at 80 weeks in TRIUMPH-1 using the efficacy estimand
• Difference: retatrutide approaches surgical-scale percentage weight loss in trial data, but it is still investigational and not a substitute for individualized medical or surgical evaluation

Maintaining the Weight Loss

Retatrutide-specific long-term discontinuation data are not yet established in public Phase 3 readouts. Other incretin-based obesity drugs show regain after stopping, so durable treatment planning remains important.

Real-World Expectations

• Trial participants are selected (motivated, adherent)
• Retatrutide has no approved label and no post-approval real-world effectiveness data
• Do not assume trial percentages will translate directly to clinical practice
• Individual variation is significant

Limitations of Current Data

• Phase 3 results cited here are Lilly topline releases, not full peer-reviewed publications
• Long-term safety and efficacy data still accumulating
• No FDA-approved prescribing information exists yet
• Head-to-head obesity trials vs tirzepatide are not yet available

Bottom Line

Retatrutide's 24.2% result is from the Phase 2 obesity trial at 48 weeks, not TRIUMPH-1. TRIUMPH-1 Phase 3 reported 28.3% at 80 weeks with 12 mg using the efficacy estimand, and TRIUMPH-4 reported 28.7% at 68 weeks with 12 mg using the efficacy estimand in adults with obesity or overweight and knee osteoarthritis. Retatrutide is still investigational and not available by prescription.

See the retatrutide guide. Related: vs tirzepatide, dosage protocol.