Both drugs are Eli Lilly's. Tirzepatide (Zepbound, Mounjaro) is FDA-approved and prescribable today; it activates two receptors, GIP and GLP-1. Retatrutide is investigational; it adds a third mechanism — glucagon-receptor agonism, believed to raise energy expenditure — and cannot be prescribed outside clinical trials or Lilly's expanded access program.
The numbers, side by side — with the caveat that matters
These are different trials with different populations, durations and statistical methods — not a head-to-head. "Efficacy estimand" measures the effect if participants stay on treatment; "treatment-regimen estimand" counts everyone regardless of discontinuation, and runs lower. Compare like with like, and treat cross-trial gaps as indicative only. The real head-to-head is TRIUMPH-5, below.
| Retatrutide — TRIUMPH-1 [1] | Tirzepatide — SURMOUNT-1 [2] | |
|---|---|---|
| Status | Investigational (submission planned Q1 2027) | FDA-approved |
| Duration | 80 weeks | 72 weeks |
| Top-dose result (efficacy estimand) | 28.3% (12 mg) | 22.5% (15 mg) |
| Treatment-regimen estimand | ~25.0% top dose, as reported by the Pharmaceutical Journal [6] | ~20.9% (15 mg) |
| ≥30% weight loss | 45.3% of the 12 mg group | — |
| Notable safety signal | GI events; dysesthesia (12.3–12.5% on higher doses vs 0.9% placebo) [5] | GI events |
For context on tirzepatide's own comparative strength: in SURMOUNT-5 it beat semaglutide directly, 20.2% vs 13.7%. [3]
TRIUMPH-5: the comparison that will actually settle it
ProjectedTRIUMPH-5 (NCT06662383) is the head-to-head trial of retatrutide versus tirzepatide. Completion is expected around December 2026 — a third-party estimate, not a commitment. [4] Until it reads out, no one can honestly say retatrutide "beats" tirzepatide.
The practical difference today
Tirzepatide can be prescribed now; retatrutide cannot. If you're deciding whether to start an approved medication or wait, that's a conversation for a licensed clinician — see Should I wait for retatrutide? and the practices in the RetaMaps directory that advertise tirzepatide today.