TL;DR: TRIUMPH-1, retatrutide’s first reported phase 3 obesity trial, put mean weight loss at the 12 mg dose at roughly 28.3% (about 31.9 kg) over 80 weeks in adults without diabetes, with 45.3% of participants crossing the 30% threshold. The real story is what came after: a blinded 104-week extension in people with a starting BMI of 35 or higher reached about 30.3%, and the curve reportedly was still descending. Every prior GLP-1 trial we’ve charted flattens well before two years. This one, on topline numbers alone, had not.
TRIUMPH-1, in brief
On 21 May 2026, Eli Lilly reported topline results from TRIUMPH-1, a phase 3, randomized, double-blind, placebo-controlled trial of retatrutide in roughly 2,339 adults with obesity or overweight and at least one weight-related condition, none of whom had diabetes. Participants were randomized to weekly retatrutide at 4 mg, 9 mg, or 12 mg, or to placebo, over an 80-week primary treatment period.
All three doses met the trial’s primary and key secondary endpoints. That is the routine part. What made TRIUMPH-1 worth a second look is a smaller, pre-specified extension: participants with a baseline BMI of 35 or higher who tolerated their assigned dose continued into a blinded extension out to 104 weeks total, on a path to their maximum tolerated dose. That extension is where the plateau question gets interesting, and we’ll get there. First, the topline numbers.
The headline number: 28.3% at 12 mg, and 45.3% past 30%
At 80 weeks, mean weight change on 12 mg retatrutide was about -28.3%, or roughly 31.9 kg, against about -2.2% (roughly 2.5 kg) on placebo. The 9 mg and 4 mg arms reported roughly -25.9% and -19.0%. Beyond the average, the distribution numbers are arguably more telling: about 45.3% of participants on 12 mg lost at least 30% of body weight, a threshold that has historically only shown up after bariatric surgery, and about 65.3% brought their BMI under 30 by week 80.
TRIUMPH-1 (retatrutide, 80 wk, topline), SURMOUNT-1 (tirzepatide, 72 wk), STEP 1 (semaglutide, nadir near wk 60). Different trials, durations, and populations — not a head-to-head comparison.
For context on where those other two numbers come from, our breakdown of why GLP-1 weight loss plateaus charts the same semaglutide and tirzepatide figures against the week each trial reached its nadir. That comparison is exactly why the extension data below is the part worth reading closely.
The curve that didn’t plateau
Here is the finding that separates TRIUMPH-1 from every other incretin trial we’ve covered. Among the subset of participants with baseline BMI at or above 35 who continued into the blinded 104-week extension on 12 mg, mean weight loss reportedly reached about 30.3%, or roughly 38.6 kg. That’s not a huge jump over the week-80 number, roughly two additional percentage points, but the shape of the curve is what matters. Lilly’s release described the trajectory as continuing to descend through week 104 rather than leveling off.
Set that against the pattern our own plateau article documents. STEP 1’s semaglutide curve reaches its nadir around week 60 of a 68-week trial and holds from there. SURMOUNT-1’s tirzepatide curve plateaus earlier still: a dedicated analysis found a median time to plateau of 24 to 36 weeks, with roughly 88 to 90% of participants flattened out by week 72. Both of those are large, published, peer-reviewed phase 3 datasets, and both plateau well inside the first 18 months. A curve still moving at week 104, two full years in, would be a genuinely different shape.
Illustrative shape only, not raw trial data. Retatrutide line approximates the reported 28.3% at week 80 and 30.3% at week 104 in the BMI 35+ extension; the comparison line approximates a typical GLP-1 plateau shape (semaglutide, STEP 1).
There’s a mechanistic story that would explain a slower plateau if it holds: retatrutide adds glucagon-receptor activity on top of the GIP and GLP-1 agonism shared with tirzepatide, and glucagon signaling is associated in earlier research with higher energy expenditure. More energy burned at a given body size could, in theory, push the point where intake and expenditure re-equalize further out in time. That’s a plausible mechanism, not a proven one. Our retatrutide vs. tirzepatide comparison goes through the receptor biology in more depth if you want the fuller picture, and the primer on what retatrutide is covers the compound from the ground up.
Tolerability: mostly GI, and more dropout at the top dose
The adverse-event pattern in TRIUMPH-1 looks like every other incretin trial in this class: nausea, diarrhea, vomiting, constipation, and reduced appetite, concentrated during dose escalation and mostly mild to moderate. Discontinuation for adverse events rose with dose, which is the expected shape for a titrated peptide.
No severe hypoglycemia was reported. A small number of deaths occurred during the trial, reported as unrelated to treatment by investigators. None of that is unusual for this drug class at this scale; it reads as consistent with the tolerability profile already seen in retatrutide’s phase 2 trial and in tirzepatide’s phase 3 program. If you’re weighing where a given dose sits on the escalation path, our retatrutide dosing and titration walkthrough lays out the standard schedule.
How this stacks up against tirzepatide and semaglutide
Cross-trial percentages are always an imperfect comparison, since duration, dose titration, and baseline population all differ. With that caveat stated plainly: TRIUMPH-1’s 28.3% at 12 mg sits numerically above SURMOUNT-1’s 22.5% at tirzepatide’s top dose and well above STEP 1’s 14.9% nadir on semaglutide 2.4 mg. Our three-way GLP-1 comparison across semaglutide, tirzepatide, and retatrutide lines up the fuller dataset, including the phase 2 numbers that preceded TRIUMPH-1.
What’s new here isn’t just the bigger number. Retatrutide’s own phase 2 trial already put 12 mg at 24.2% over 48 weeks, so a topline of 28.3% over a longer 80-week trial is roughly the trajectory you’d expect. The extension curve is the part without a clean precedent: nothing in the semaglutide or tirzepatide literature shows a mean weight-loss line still moving at the two-year mark. If TRIUMPH-1’s extension holds up in the published data, it would be the first phase 3 incretin trial where “how long do you keep losing” doesn’t have a one-year answer.
What’s still unconfirmed
A few things are worth holding loosely until more data lands. The 104-week figure comes from a subset, participants with baseline BMI 35 or higher who both tolerated their dose and opted into the extension, which is a narrower and likely more treatment-responsive group than the full 80-week cohort. Selection effects like that can make a subgroup’s curve look better than the trial’s true population average. The “no plateau” framing is also a topline characterization rather than a reported statistical test, so we don’t yet know how flat or steep the curve actually was near the end, only that it hadn’t leveled off by week 104 on the numbers released so far. And the full 80-week manuscript, with per-arm confidence intervals and a complete adverse-event table, hasn’t published yet.
None of that erases the finding. It just means the honest version of this story is “reported, striking, and awaiting confirmation” rather than “proven.”
Frequently asked questions
What were retatrutide’s phase 3 TRIUMPH-1 results?
At 80 weeks, mean weight loss on retatrutide 12 mg was about 28.3% (roughly 31.9 kg) versus about 2.2% on placebo, in a trial of about 2,339 adults without diabetes. About 45.3% of the 12 mg group lost at least 30% of body weight, and about 65.3% brought their BMI under 30. The 9 mg and 4 mg arms reported roughly 25.9% and 19.0%.
Did the retatrutide weight-loss curve really not plateau?
In the reported 104-week blinded extension, limited to participants with a baseline BMI of 35 or higher, mean weight loss reached about 30.3% and the curve was described as still descending rather than flat. That’s a topline characterization from a press release, not a peer-reviewed statistical finding, so treat the “no plateau” claim as a strong lead rather than a settled result until the full manuscript publishes.
How does retatrutide’s 28.3% compare to tirzepatide and semaglutide?
Numerically, 28.3% at retatrutide’s 12 mg dose is higher than tirzepatide’s 22.5% nadir on 15 mg (SURMOUNT-1) and semaglutide’s 14.9% nadir on 2.4 mg (STEP 1). These are cross-trial comparisons across different durations and populations, not a head-to-head trial, so the gap should be read as directional rather than exact.
What were the most common side effects in TRIUMPH-1?
Gastrointestinal effects dominated: nausea, diarrhea, vomiting, constipation, and reduced appetite, mostly mild to moderate and concentrated during dose escalation. Discontinuation for adverse events rose with dose, from about 4.1% at 4 mg to about 11.3% at 12 mg, versus about 4.9% on placebo. No severe hypoglycemia was reported.
Is TRIUMPH-1 published yet, or just a press release?
As of this writing, TRIUMPH-1’s numbers come from a sponsor topline release and conference-style summaries, not a peer-reviewed manuscript. The headline figures are consistent with retatrutide’s earlier phase 2 trial, which is reassuring, but the full statistical detail, subgroup breakdowns, and safety tables typically arrive with the published paper.
Why does a non-plateauing curve matter?
Every large GLP-1 obesity trial published so far shows weight loss flattening within the first 12 to 18 months as the body’s energy balance re-equalizes at a lower weight; our plateau article walks through why that happens. A curve still descending at two years would be a first for this drug class, and it’s the main reason TRIUMPH-1 is worth tracking past its topline headline.