Retatrutide vs. Tirzepatide: What the Trials Actually Found

Why these numbers are not a comparison

A table with three rows next to each other invites a reader to do the subtraction anyway, so it is worth being explicit about what makes these three results incomparable. They come from three different trials, run by different research groups, in different phases of development, on different patient populations, for different durations, at three very different sample sizes — 338, 537, and 2,539 randomized participants. A trial testing a drug for 72 weeks in people without diabetes is not measuring the same thing as a trial testing a different drug for 40 weeks in people who have it. Stacking the topline percentages into a bar chart would flatten all of that away and leave only the one number that looks most dramatic, which is exactly the comparison this page declines to draw.

What retatrutide's trials found

Retatrutide's phase 2 obesity trial randomized 338 adults without diabetes to placebo or one of several retatrutide doses and followed them for 48 weeks. At the 12 mg dose, average body weight fell 24.2%, against 2.1% for placebo ¹. That is the number most often quoted as retatrutide's headline result, and it is a real, randomized, placebo-controlled finding — but it is a phase 2 result, run in fewer than 350 people, in a population that excluded anyone with diabetes.

Retatrutide's phase 3 program has since reported its first result. TRANSCEND-T2D-1 tested retatrutide as a monotherapy in 537 adults who have type 2 diabetes inadequately controlled by diet and exercise alone — not the obesity-only population the phase 2 trial enrolled — at 48 sites in the USA, Mexico and India, over 40 weeks. Its primary endpoint was change in HbA1c, which fell 1.94 percentage points on 12 mg against 0.81 on placebo; weight change was a key secondary endpoint, and fell 15.3% on 12 mg against 2.6% on placebo ².

Three things differ at once between that 15.3% and the phase 2 trial's 24.2%, and the gap cannot be apportioned between them. The population differs — type 2 diabetes versus obesity without diabetes. The duration differs — 40 weeks versus 48. And the analysis convention differs: TRANSCEND-T2D-1 states that its figures are the treatment regimen estimand, which keeps a participant's data in the analysis after they stop the study drug ², where the phase 2 trial reports a least-squares mean and names no estimand at all ¹. An estimand that counts post-discontinuation data reads lower than one that does not, independent of how the drug performed. So the smaller phase 3 number is not a measurement of retatrutide getting weaker, and it is equally not a like-for-like confirmation of the phase 2 number at a lower value. It is a different question, answered differently.

What tirzepatide's trial found

Tirzepatide's phase 3 approval trial, SURMOUNT-1, randomized 2,539 adults with obesity and no diabetes to placebo or tirzepatide and followed them for 72 weeks — closer to a year and a half, and roughly seven times the enrollment of retatrutide's phase 2 trial. At the 15 mg dose, average body weight fell 20.9%, against 3.1% for placebo ³. That is the trial regulators approved tirzepatide on, and it remains the largest, longest, and latest-phase result of the three in this table.

Availability is a separate question from evidence

Evidence and availability answer different questions, and this page keeps them separate on purpose. Tirzepatide is FDA-approved and sold by prescription under the brand names Zepbound and Mounjaro — a reader who wants it can get it through a licensed prescriber and pharmacy today. Retatrutide is not approved for any use, so no prescriber can write for it and no pharmacy can fill it. The two pre-approval routes that do exist are enrollment in a trial and a single-patient expanded-access program requested by a treating physician, listed on ClinicalTrials.gov as NCT07629401; neither is a commercial channel and neither is something a reader arranges for themselves. Material sold under the name outside those routes is unregulated stock with none of the manufacturing or purity oversight an approved drug carries. A published phase 3 does not change any of that — a reported trial and an approved medicine are different things, and retatrutide is still only the first.

The bottom line

Read this table as three separate trial reports, not a race. Retatrutide's phase 2 result is the largest number here, from the smallest and shortest trial, in a population that excluded people with diabetes. Retatrutide's own phase 3 result is smaller, but it was measured in a diabetes population, over 40 weeks, under a different estimand — so it belongs beside tirzepatide's number as a third separate finding, not underneath it as a defeat. Tirzepatide's number is the one backed by the largest, longest, latest-phase trial in this table, and it is the only one of the three attached to a drug a reader can actually be prescribed. The question of which molecule produces more weight loss in the same people over the same weeks is the one NCT06662383 was registered to answer, and as of August 2026 it has not reported.