Retatrutide vs Zepbound: Investigational Against Approved
Retatrutide (retatrutide (LY3437943), Eli Lilly) vs Zepbound (tirzepatide, Eli Lilly)
Last verified July 2026
The verdict
Retatrutide's −24.2% at 48 weeks came from an 81-person phase 2 arm; Zepbound's −20.9% at 72 weeks came from a 630-person phase 3 arm. The headline gap is smaller than the evidence gap. Retatrutide is not approved, cannot be prescribed, and what is sold under its name online is unregulated. Zepbound is available at $349–$499 through LillyDirect. This is a comparison worth understanding and not, today, a decision.
Side-by-side comparison
| Field | Retatrutide | Zepbound |
|---|---|---|
| Peak weight loss (trial) | −24.2% at 48 wk (Phase 2, 12 mg; N=81 in highest-dose arm) | −20.9% at 72 wk (SURMOUNT-1, 15 mg; Phase 3, N=630 in that arm) |
| Mechanism | Triple agonist: GLP-1, GIP, and glucagon receptors | Dual agonist: GLP-1 and GIP receptors |
| FDA status | Not FDA-approved; TRIUMPH Phase 3 trials ongoing | FDA approved November 2023 for chronic weight management in adults |
| Trial stage | Phase 3 (TRIUMPH program; results expected 2026–2027) | Phase 3 complete (SURMOUNT program); full regulatory approval granted |
| Availability | Not available; accessible through TRIUMPH clinical trials (clinicaltrials.gov) only | Commercially available by prescription in the United States |
| Administration | Once-weekly subcutaneous injection | Once-weekly subcutaneous injection (2.5 mg–15 mg titrated dose) |
| GI side effects | Dose-related nausea, vomiting, diarrhea; mostly mild–moderate in Phase 2; dose-dependent heart rate increase (peaked wk 24, declined thereafter) | Nausea, vomiting, diarrhea, constipation; mild–moderate, mainly during dose escalation; discontinuation rate 4.3–7.1% |
What each one costs in the compounded market
Standing monthly cash prices from the 556 sellers on this register. These are compounded preparations, not the approved products compared above.
Retatrutide — retatrutide (LY3437943)
No seller on this register publishes a compounded price for it. For a drug still in trials that is expected — and anyone selling it anyway is doing so outside the system this register covers.
Zepbound — tirzepatide
from $99 /mo
253 sellers publish a standing price; the median is $240 a month.
⚠ A compounded preparation is not the approved product. No agency has reviewed that specific preparation for safety, effectiveness or manufacturing quality, and the legal ground under compounding these molecules narrowed when the shortages closed. The price gap is real, and so is the difference in oversight. Every seller, with its price record.
Frequently asked questions
Which works better for weight loss, Retatrutide or Zepbound?
Retatrutide recorded −24.2% at 48 wk (Phase 2, 12 mg; N=81 in highest-dose arm); Zepbound recorded −20.9% at 72 wk (SURMOUNT-1, 15 mg; Phase 3, N=630 in that arm). ⚠ These come from separate trials with different protocols and populations, so the gap between two numbers from two studies is weaker evidence than it looks. No head-to-head trial of this pair is on record here.
Are Retatrutide and Zepbound the same drug?
No. Retatrutide is retatrutide (LY3437943) and Zepbound is tirzepatide — different molecules, and they work differently: Triple agonist: GLP-1, GIP, and glucagon receptors against Dual agonist: GLP-1 and GIP receptors. Results with one do not predict results with the other.
Which has worse side effects, Retatrutide or Zepbound?
The most common adverse event reported was Dose-related nausea, vomiting, diarrhea; mostly mild–moderate in Phase 2; dose-dependent heart rate increase (peaked wk 24, declined thereafter) for Retatrutide and Nausea, vomiting, diarrhea, constipation; mild–moderate, mainly during dose escalation; discontinuation rate 4.3–7.1% for Zepbound. ⚠ Rates come from separate trial populations and are not a like-for-like comparison. In practice how fast the dose is escalated affects tolerability at least as much as which drug it is, which is one reason a slower titration is worth asking for.
How is each one taken?
Retatrutide: Once-weekly subcutaneous injection. Zepbound: Once-weekly subcutaneous injection (2.5 mg–15 mg titrated dose). Worth weighing honestly against your own habits — the best drug for you is partly the one you will still be taking in eight months, and a dosing routine that does not fit a life gets abandoned.
What does a compounded version of each actually cost?
Across the 556 sellers in this register, the cheapest standing monthly cash prices we hold are tirzepatide from $99 a month. ⛔ A compounded preparation is not the approved product: no agency has reviewed that specific preparation for safety, effectiveness or manufacturing quality, and the legal ground under compounding these molecules narrowed once the shortages closed. The price gap is real and so is the difference in oversight.
Can I switch from Retatrutide to Zepbound?
Yes, with a prescriber. Expect to re-titrate from the bottom of the new drug's ladder rather than starting where you left off: these are different molecules and tolerance to one does not transfer to the other. Both the old drug's washout and the new drug's schedule apply.
Which one should I ask for?
That is a prescriber's call, and anyone answering it from a table is guessing. What a page like this can do is tell you what the trials measured, what the manufacturers charge, and where the evidence is still missing, so the conversation starts from the record rather than from an advertisement.
References
- 1.Jastreboff AM, Kaplan LM, Frías JP, et al.; Retatrutide Phase 2 Obesity Trial Investigators Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial N Engl J Med. 2023. PMID: 37366315.
- 2.Jastreboff AM, Aronne LJ, Ahmad NN, et al.; SURMOUNT-1 Investigators Tirzepatide Once Weekly for the Treatment of Obesity N Engl J Med. 2022. PMID: 35658024.
- 3.Giblin K, Bethel MA, et al. Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: Rationale and design of the TRIUMPH registrational clinical trials Diabetes Obes Metab. 2026. PMID: 41090431.
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