Dual agonist vs triple agonist. Both from Eli Lilly. One's available, one could be the endgame.
Bottom line: Both are Eli Lilly drugs, and retatrutide is almost certainly meant to be tirzepatide's successor. The ~29% weight loss in Phase 2 vs tirz's ~21% is a meaningful gap. But retatrutide has no approval, no commercial supply, and only Phase 2 data. Tirzepatide is the best available option right now. Retatrutide might be the best option in 2–3 years.
| Tirzepatide | Retatrutide | |
|---|---|---|
| Category | Dual GLP-1/GIP receptor agonist | Triple GLP-1/GIP/glucagon receptor agonist |
| Mechanism | Activates GLP-1 and GIP receptors for appetite suppression and improved insulin sensitivity | Adds glucagon receptor activation on top — unlocks direct fat metabolism and liver fat reduction |
| Reported dose | 2.5–15 mg weekly | 1–12 mg weekly (Phase 2 doses) |
| Frequency | Once weekly | Once weekly |
| Weight loss (trials) | ~20–22% at 72 weeks (SURMOUNT, 15 mg) | ~24.2% at 48 weeks, up to 28.7% at 68 weeks (Phase 2, 12 mg) |
| Top side effect | Nausea, diarrhea, vomiting — GI symptoms during titration | Similar GI profile, potentially higher rates at maximum doses |
| Evidence tier | Tier 1 — Phase 3 complete, FDA-approved | Tier 3 — Phase 2 complete, Phase 3 ongoing |
| Common vial size | 5–15 mg vials (compounded); branded pens pre-filled | Not commercially available |
| Approval status | FDA-approved as Mounjaro (diabetes) and Zepbound (obesity) | Investigational — Eli Lilly Phase 3 in progress |
Tirzepatide is the practical choice because it exists and you can get it today. It already produces more weight loss than semaglutide and has a solid safety profile established through large Phase 3 trials and millions of real-world prescriptions.
Retatrutide is the theoretical ceiling — if Phase 3 confirms the Phase 2 data, the glucagon receptor activation could produce weight loss numbers that make even tirzepatide look modest. The liver fat reduction data is particularly interesting. But this is a 'watch this space' situation, not a 'go get this' situation.
Both are Eli Lilly drugs, and retatrutide is almost certainly meant to be tirzepatide's successor. The ~29% weight loss in Phase 2 vs tirz's ~21% is a meaningful gap. But retatrutide has no approval, no commercial supply, and only Phase 2 data. Tirzepatide is the best available option right now. Retatrutide might be the best option in 2–3 years.
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vialprep research · Sources: PubMed, FDA, peer-reviewed trials · How we verify