Retatrutide vs Tirzepatide
Last updated September 26, 2026 · Reviewed by Grey Peptides Editorial Board · ✓ Primary-sourced
← Back to the Retatrutide Hub · See also: Dosage · FDA approval status
The core difference is one receptor. Tirzepatide is a dual agonist (GLP-1 + GIP); retatrutide is a triple agonist that adds glucagon. That third lever raises energy expenditure and liver-fat clearance, and it shows up in the numbers: retatrutide reached up to 28.7% mean weight loss in Phase 3 versus tirzepatide's ~22.5% in its pivotal obesity trial.
But the comparison isn't apples-to-apples: there's no head-to-head trial, retatrutide is not approved, and its long-term safety is unproven (including a new dysesthesia signal). Tirzepatide is the approved, prescribable efficacy leader available today.
→ Build your own side-by-side across 12 metrics in the Comparison Tool.
Quick comparison
| Feature | Retatrutide | Tirzepatide |
|---|---|---|
| Receptors | GLP-1 + GIP + glucagon (triple) | GLP-1 + GIP (dual) |
| Brand names | None (investigational, "LY3437943") | Mounjaro (T2D), Zepbound (obesity) |
| Status (2026) | Phase 3, not approved | FDA approved |
| Route | Weekly subcutaneous injection | Weekly subcutaneous injection |
| Peak trial weight loss | ~28.7% (TRIUMPH-4, 12 mg, 68 wks); 28.3% (TRIUMPH-1, 12 mg, 80 wks) | ~22.5% (SURMOUNT-1, 15 mg, 72 wks) |
| Max studied dose | 12 mg/week | 15 mg/week |
| Distinct effect | Glucagon: energy expenditure + liver-fat oxidation | GIP: fat-tissue insulin sensitivity |
| Notable safety note | New dysesthesia signal (dose-dependent) | Established GI profile; CV outcome trial ongoing |
| Availability | Clinical trials; narrow expanded access (as of September 26, 2026) | Prescription (with supply/access caveats) |
| Typical monthly cost | Not priced (not approved) | Varies by brand and channel; dated prices on the GLP-1 cost page |
The mechanism difference
Both drugs are single molecules engineered to hit multiple receptors at once. Tirzepatide combines GLP-1 (appetite suppression, slowed gastric emptying, better insulin response) with GIP (improved fat-tissue insulin handling, possibly gentler on nausea). Retatrutide keeps both of those and adds glucagon-receptor agonism.
Glucagon is the differentiator. At the liver and in fat tissue it increases energy expenditure and drives fat oxidation — so retatrutide doesn't only reduce how much you eat, it appears to increase how much energy the body burns and how aggressively it clears liver fat. That's why, in trials, retatrutide's liver-fat and triglyceride reductions outpaced what weight loss alone would predict. The trade-off is added complexity: the glucagon component must be balanced through slow titration so it doesn't push blood sugar the wrong way. See the full breakdown on the hub.
Weight-loss data
The honest caveat first: no trial has compared these two drugs directly. What we have is their separate pivotal data, which used different populations and durations, so cross-trial comparisons are suggestive, not definitive.
Tirzepatide's SURMOUNT-1 produced about 22.5% mean weight loss at the 15 mg dose over 72 weeks. Retatrutide's Phase 2 reached 24.2% at 48 weeks, and its first Phase 3 readout (TRIUMPH-4) reached up to 28.7% at 68 weeks — the largest figure reported in any obesity trial to date. The pivotal obesity trial, TRIUMPH-1, reported 28.3% at 80 weeks (Lilly, May 21, 2026). On magnitude alone, retatrutide leads. But tirzepatide has years of real-world use, a maturing safety record, and an ongoing cardiovascular outcomes trial; retatrutide has none of those yet.
→ For the two approved leaders compared in depth, see Semaglutide vs Tirzepatide.
Side effects
Both share the incretin-class gastrointestinal profile — nausea, diarrhea, constipation, vomiting, reduced appetite — worst during dose escalation. Retatrutide's Phase 3 data added a new wrinkle: dysesthesia (an abnormal skin sensation) appeared in a dose-dependent way (about 8.8% at 9 mg and 20.9% at 12 mg vs 0.7% on placebo). It was generally mild and rarely caused discontinuation, but it wasn't seen with tirzepatide and is being watched. Tirzepatide's side-effect profile, by contrast, is well characterized after years on the market.
Which one matters for you?
If you need something available and proven today, tirzepatide is the approved option with the deeper track record. If you're tracking where the field is heading, retatrutide is the higher-ceiling drug — but it's still in trials, and "higher trial numbers" is not the same as "better for a given person," which depends on tolerability, comorbidities, cost, and clinical judgment. Neither is a substitute for a conversation with a prescriber.
Frequently asked questions
Is retatrutide just "stronger Mounjaro"?
Roughly, in terms of weight-loss magnitude — but it's a different molecule with a third receptor and its own safety profile, not a higher dose of tirzepatide.
Will retatrutide replace tirzepatide?
Too early to say. Its Phase 3 package for obesity is in, but it still needs FDA approval (Lilly plans to file in Q1 2027) and a long-term safety record (as of September 26, 2026). Even then, both could coexist for different patients.
Can I switch from tirzepatide to retatrutide?
Not outside a clinical trial or Lilly's narrow expanded-access program — retatrutide isn't approved or prescribable (as of September 26, 2026). Any switch between approved drugs is a clinical decision for your prescriber.
Lilly's other new obesity programme, the amylin agonist eloralintide and its combination with tirzepatide, is compared with retatrutide in eloralintide and EloraTZP vs retatrutide.
Sources
- Jastreboff AM, et al. Retatrutide Phase 2 obesity trial. N Engl J Med. 2023;389(6):514–526.
- Eli Lilly & Company. TRIUMPH-4 Phase 3 topline. News release, December 11, 2025.
- Jastreboff AM, et al. SURMOUNT-1 (tirzepatide). N Engl J Med. 2022;387:205–216.
- Eli Lilly & Company. TRIUMPH-1 Phase 3 topline results (obesity, 80 weeks). News release, May 21, 2026.
- Eli Lilly & Company. TRIUMPH-2 and TRIUMPH-3 Phase 3 topline results; Biologics License Application planned for Q1 2027. News release, July 23, 2026.
Medical disclaimer: Education only, not medical advice. Retatrutide is investigational and not approved for use. Consult a licensed clinician.