Retatrutide vs. Tirzepatide: What the Trials Actually Show

Put retatrutide's headline weight-loss figure next to tirzepatide's and retatrutide wins by a wide margin. That comparison is everywhere, and it is built on a move that does not survive inspection: taking a percentage from one trial and setting it beside a percentage from a completely different trial, run in different years, on different people, for different lengths of time, and reading the gap as if it meant something.

It does not mean nothing. It also is not a result. This article does the version that is actually defensible — reporting each trial's own number with its own participant count, duration, population, and publication status attached, and then being explicit about which of those differences make the side-by-side unfair. There is no winner declared at the end, because the study that could declare one has not reported yet.

The head-to-head trial that would settle this exists — and it has reported nothing

Start with the fact that reframes everything below. A live search of ClinicalTrials.gov for a trial containing both drugs returns TRIUMPH-5 (NCT06662383): "A Phase 3, Randomized, Double-Blind Study to Evaluate the Efficacy and Safety of Retatrutide Compared to Tirzepatide in Adults Who Have Obesity," sponsored by Eli Lilly — which happens to make both molecules. Two arms, retatrutide and tirzepatide. One primary outcome: percent change from baseline in body weight. Roughly 800 participants, enrollment started November 2024, status "active, not recruiting," with primary completion estimated for November 2026⁹.

No results have been posted. That is the whole status: the experiment that could answer "which of these two produces more weight loss in the same people, at the same time, under the same protocol" is running right now and has published nothing. Until it does, every retatrutide-versus-tirzepatide percentage in circulation — including every number in this article — is a cross-trial comparison, which is a fundamentally weaker kind of evidence. Our tirzepatide-versus-semaglutide article shows the contrast directly: that question does have completed randomized head-to-head data, and it reads very differently from what follows here.

Each trial's own numbers, with its own N and duration attached

Six trials, reported one at a time rather than pooled into a ranking:

Retatrutide, Phase 2, obesity (2023). A double-blind, randomized, placebo-controlled Phase 2 trial enrolled 338 adults with a BMI of 30 or higher, or 27 to under 30 plus at least one weight-related condition, and ran for 48 weeks. Its own reported result: least-squares mean weight change at 48 weeks of -8.7% on 1 mg, -17.1% on the combined 4 mg groups, -22.8% on the combined 8 mg groups, and -24.2% on 12 mg, against -2.1% for placebo¹. Worth noting because it is routinely misquoted: the trial's primary endpoint was the change at 24 weeks, where the 12 mg figure was -17.5%. The famous -24.2% is a secondary endpoint at the trial's end.

Retatrutide, Phase 2, type 2 diabetes (2023). 281 adults with type 2 diabetes, randomized across placebo, dulaglutide 1.5 mg, and six retatrutide arms. HbA1c fell 2.02 percentage points at 24 weeks on the 12 mg arm, against -0.01% on placebo and -1.41% on dulaglutide; bodyweight fell 16.94% at 36 weeks on that same arm, against 3.00% on placebo².

Retatrutide, Phase 3, type 2 diabetes — TRANSCEND-T2D-1 (2026). This is the only completed retatrutide Phase 3 trial with a full peer-reviewed manuscript. 537 adults with type 2 diabetes inadequately controlled by diet and exercise, randomized 1:1:1:1 across three retatrutide doses and placebo, 40 weeks, at 48 sites in the USA, Mexico, and India. HbA1c fell 1.94 percentage points on 12 mg against 0.81 on placebo; bodyweight fell 15.3% against 2.6%³.

Retatrutide, Phase 3, obesity — TRIUMPH-1. The trial behind retatrutide's biggest headline numbers, and the one where the evidentiary standard drops. ClinicalTrials.gov records it as completed, with an actual enrollment of 2,335 and an actual primary completion date of April 6, 2026¹⁰. Eli Lilly's own topline announcement reports weight change at 80 weeks of -19.0% on 4 mg, -25.9% on 9 mg, and -28.3% on 12 mg, against -2.2% for placebo, from an average baseline of 112.7 kg and a BMI of 40.0; a pre-specified extension enrolling 532 participants with a baseline BMI of 35 or higher reached -30.3% at 104 weeks on the top dose⁸. Those are the numbers everyone quotes. They exist only as a press release. A live PubMed search returns no peer-reviewed TRIUMPH-1 results paper — what has been published is a design paper describing how the TRIUMPH trials were set up, not what any of them found¹¹ — and a company announcement is not the same evidentiary object as a manuscript that went through peer review and carries a full results table. A small illustration of why that matters: Lilly's release states the trial "randomized 2,339 participants," while ClinicalTrials.gov's own record lists an actual enrollment of 2,335¹⁰. Nothing turns on four people, but a peer-reviewed manuscript is where a participant-flow diagram would reconcile the two, and none exists to check.

Tirzepatide, Phase 3, obesity — SURMOUNT-1 (2022). 2,539 adults with a BMI of 30 or more, or 27 or more with at least one weight-related complication, excluding diabetes, randomized to three tirzepatide doses or placebo for 72 weeks. Mean weight change at week 72: -15.0% on 5 mg, -19.5% on 10 mg, -20.9% on 15 mg, against -3.1% on placebo, from a baseline mean weight of 104.8 kg and BMI of 38.0⁴.

Tirzepatide, Phase 3, obesity with type 2 diabetes — SURMOUNT-2 (2023). 938 adults with obesity and type 2 diabetes, 72 weeks. Least-squares mean weight change: -12.8% on 10 mg and -14.7% on 15 mg, against -3.2% on placebo, from a baseline weight of 100.7 kg, BMI 36.1, and HbA1c 8.02%⁵.

Why lining those numbers up is not a fair fight

Every one of the differences below is real, documented in the trials' own methods, and would need to be neutralized before a percentage gap could be attributed to the molecules rather than to the studies.

A Phase 2 trial and a Phase 3 trial are not the same instrument. Retatrutide's obesity data with the largest published, peer-reviewed footprint is a 338-person Phase 2 dose-finding study. Tirzepatide's is a 2,539-person Phase 3 registrational trial. Phase 2 studies exist to select doses and estimate effect size; they are smaller, more variable, and systematically prone to producing effect estimates that shrink when the same drug reaches Phase 3. Setting one against the other is comparing an estimate to a measurement.

The durations do not match. 48 weeks (retatrutide Phase 2), 40 weeks (TRANSCEND-T2D-1), 72 weeks (both SURMOUNT trials), 80 weeks and a 104-week extension (TRIUMPH-1). Weight-loss curves in this drug class are still descending at 48 weeks. Comparing a 48-week number to a 72-week number understates the shorter trial; comparing an 80-week number to a 72-week number overstates the longer one. Neither direction is a molecular difference.

The populations differ, and their baselines prove it. TRIUMPH-1's participants started at an average weight of 112.7 kg and a BMI of 40.0. SURMOUNT-1's started at 104.8 kg and a BMI of 38.0. That is a meaningfully heavier trial population, and percent-of-body-weight endpoints do not behave identically across baseline weights. SURMOUNT-2 and both retatrutide diabetes trials add another axis entirely: people with type 2 diabetes consistently lose less weight on this drug class than people without it — visible inside tirzepatide's own program, where 15 mg produced -20.9% in SURMOUNT-1 and -14.7% in SURMOUNT-2, the same drug at the same dose for the same 72 weeks, differing only by whether participants had diabetes.

Even the placebo arms disagree. -2.1% in retatrutide's Phase 2, -2.2% in TRIUMPH-1, -2.6% in TRANSCEND-T2D-1, -3.1% in SURMOUNT-1, -3.2% in SURMOUNT-2. Those placebo groups received no active drug, and they still differ by more than a full percentage point. That spread is the diet, exercise, and lifestyle support built into each protocol, plus population differences — a direct measurement of how much these trials differ from each other before either drug is administered.

The evidence is not equally reviewed. Tirzepatide's numbers come from published, peer-reviewed trials that FDA subsequently reviewed as part of approving the drug. Retatrutide's largest numbers come from a press release. Its one completed peer-reviewed Phase 3 trial is in type 2 diabetes, not obesity, and is a 40-week study.

One of them you can actually get. Tirzepatide is FDA-approved and marketed as Mounjaro and Zepbound⁷. Retatrutide is not approved for anything, anywhere in the United States — our approval-status article documents that in full, including what "buy retatrutide" searches actually lead to. A trial percentage attached to a molecule you cannot legally obtain is a different kind of fact from one attached to a prescription you can fill.

What a real head-to-head looks like, for contrast

It is worth seeing the alternative, because it makes the gap in this comparison concrete. SURMOUNT-5 randomized 751 adults with obesity but without type 2 diabetes to either tirzepatide or semaglutide, each titrated to that participant's own maximum tolerated dose, in the same trial, at the same time, for the same 72 weeks. Result: -20.2% with tirzepatide versus -13.7% with semaglutide, P<0.001, with the trial's own conclusion stating that tirzepatide "was superior to treatment with semaglutide with respect to reduction in body weight and waist circumference at week 72⁶."

That 6.5-point gap is attributable to the drugs, because everything else was held constant by randomization. No equivalent sentence can honestly be written about retatrutide and tirzepatide today. TRIUMPH-5 is designed to produce exactly that sentence, and it has not produced it yet.

The bottom line

Retatrutide's published Phase 2 obesity trial reported -24.2% at 48 weeks in 338 people; its unpublished Phase 3 topline reports -28.3% at 80 weeks in 2,335. Tirzepatide's Phase 3 obesity trial reported -20.9% at 72 weeks in 2,539 people, peer-reviewed and FDA-reviewed. Those are the honest numbers, each with its own trial attached. They are not a scoreboard, and this article is not going to convert them into one: the trials differed in phase, size, duration, baseline weight, diabetes status, placebo response, and level of external review, and any one of those could account for a several-point gap on its own.

What can be said without overstating anything: retatrutide's mechanism adds glucagon-receptor activity to the GIP and GLP-1 targets tirzepatide hits, its trials so far have produced large effects, and a randomized Phase 3 comparison against tirzepatide is nearly finished. Until that trial reports, the difference between these two molecules is an open question, not a settled one. For what retatrutide's trials tested by dose, see our retatrutide dosing article; for what those same trials reported about tolerability, our retatrutide side-effects article and our tirzepatide side-effects article cover each drug separately. For the one of these two that is actually available today, our tirzepatide provider rankings grade who discloses their pricing and sourcing clearly, and our tirzepatide dosing article covers what its FDA label actually says.