The Core Mechanism Difference
Both drugs target the GLP-1 and GIP receptors. The distinction is a third receptor: the glucagon receptor (GCG), which retatrutide activates in addition to GLP-1 and GIP.
| Drug | GLP-1 | GIP | Glucagon (GCG) | Classification |
|---|---|---|---|---|
| Tirzepatide (Mounjaro / Zepbound) | ✓ | ✓ | — | Dual agonist |
| Retatrutide (LY3437943) | ✓ | ✓ | ✓ | Triple agonist |
Glucagon receptor activation adds a distinct energy expenditure pathway: increased thermogenesis, direct hepatic fat mobilization, and a separate CNS satiety signal. This is not a minor addition in principle — it is an entirely separate biological axis. Whether it produces meaningfully better clinical outcomes in a larger controlled trial is what the ongoing Phase 3 study needs to demonstrate.
Weight Loss Efficacy: What the Trials Show
The trials involved different populations, durations, and entry criteria. They are order-of-magnitude indicators — not a head-to-head result. Only a randomized head-to-head trial can establish superiority.
Tirzepatide — SURMOUNT-1 (Phase 3, 72 weeks)
| Dose | Mean weight loss |
|---|---|
| 5 mg/week | –15.0% |
| 10 mg/week | –19.5% |
| 15 mg/week | –20.9% |
| Placebo | –3.1% |
Source: Jastreboff AM et al. NEJM 2022; 387:205–216. Duration: 72 weeks.
Retatrutide — Jastreboff Phase 2 (48 weeks)
| Dose | Mean weight loss |
|---|---|
| 1 mg/week | –8.7% |
| 4 mg/week | –17.1% |
| 8 mg/week | –22.8% |
| 12 mg/week | –24.2% |
| Placebo | –2.1% |
Source: Jastreboff AM et al. NEJM 2023; 389:514–526. Duration: 48 weeks.
Side Effects: Both Are Primarily GI, With Similar Profiles
GI adverse events (nausea, diarrhea, vomiting, constipation) are the dominant side effect class for both drugs, driven primarily by GLP-1 agonism and concentrated during dose escalation. Retatrutide's rates are slightly higher, likely reflecting the added glucagon receptor activity and the fact that trial participants escalated to higher absolute doses.
| Side Effect | Tirzepatide 15 mg | Retatrutide 12 mg |
|---|---|---|
| Any GI event | ~65–70% | ~75–80% |
| Nausea | ~40% | ~45% |
| Diarrhea | ~25% | ~25–30% |
| Vomiting | ~20% | ~19% |
Approximate figures; see individual trial publications for exact percentages.
Pharmacokinetics: Very Similar Half-Lives
| Property | Tirzepatide | Retatrutide |
|---|---|---|
| Half-life | ~5 days (120 h) | ~6 days (144–165 h) |
| Dosing | Once weekly | Once weekly |
| Steady state | ~4 weeks | ~4–5 weeks |
| FDA approval | Yes (Mounjaro + Zepbound) | Not yet approved |
Approval Status and Availability
This is the most practically relevant difference as of 2026. Tirzepatide is FDA-approved and available as commercial brand-name injectable pens. Retatrutide is not approved, has no commercial form, and is available only through compounding pharmacies with a valid prescription.