Tirzepatide is the current best-evidenced obesity drug. Retatrutide is the most-discussed challenger. Phase-3 retatrutide data hasn't published yet — so this comparison is partial.
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Tirzepatide is licensed; retatrutide is in late-stage development.
| Tirzepatide | Retatrutide | |
|---|---|---|
| Receptor count | 2 (GIP + GLP-1) | 3 (GIP + GLP-1 + Glucagon) |
| Mean weight reduction | −20.9% (phase-3) | −24.2% (phase-2) |
| Trial program | SURMOUNT / SURPASS | TRIUMPH |
| Approval status | Approved 2022/2023 | No approval anywhere |
| Half-life | ~5 days | ~6 days |
Phase-3 vs phase-2 is not just a step number — it determines confidence.
Tirzepatide has multiple completed phase-3 trials with >11,000 enrolled participants. Effect sizes, safety signals, and durability are all well characterised. The compound is approved by major regulators worldwide.
Retatrutide's phase-2 data are striking — the 12mg arm reported the largest mean weight reduction ever observed for a pharmacologic agent — but phase-2 trials systematically over-state effect sizes vs subsequent phase-3 confirmation. Until TRIUMPH-3 reports, retatrutide should not be treated as if it were already established.
For any UK reader making a decision in 2026, the answer is tirzepatide. It is approved, prescribable, has a complete phase-3 evidence base, and produces the largest weight reduction of any approved compound.
Retatrutide is genuinely promising and may rewrite the rankings — but the answer to "should I be using retatrutide right now" is no, because it isn't legally available outside trials. We will revise this comparison when phase-3 data publish, expected 2026–2027.
You are making a decision today and want regulatory-grade evidence behind it.
Phase-3 data published in 2026–2027 confirm phase-2 effect sizes, regulatory approval follows, and the supply picture stabilises.
This verdict reflects the published evidence as of 14 April 2026. We update comparison pages whenever new phase-3 data is published.