GLP-1 weight loss trial results: what the studies found

Short answer

Two phase 3 trials, both published in the New England Journal of Medicine, measured average body weight change at the end of follow-up: 68 weeks in one and 72 weeks in the other. STEP 1, which studied semaglutide, reported an average change of minus 14.9 percent at 68 weeks. SURMOUNT-1, which studied tirzepatide, reported minus 15.0 percent at the 5 mg dose, minus 19.5 percent at 10 mg and minus 20.9 percent at 15 mg, at 72 weeks. In both trials the placebo arm changed far less: minus 2.4 percent in STEP 1 and minus 3.1 percent in SURMOUNT-1.

These are averages from two named trials. They are not a promise about any individual, and the sizes of the studies, the doses used and the length of follow-up differ between them.

STEP 1: semaglutide, 68 weeks

STEP 1 was published in the New England Journal of Medicine in 2021. It enrolled 1,961 adults without diabetes, and it compared semaglutide with placebo over 68 weeks. The published average body weight change was minus 14.9 percent on semaglutide and minus 2.4 percent on placebo.

SURMOUNT-1: tirzepatide, 72 weeks

SURMOUNT-1 was published in the New England Journal of Medicine in 2022. It enrolled 2,539 adults, and it compared three tirzepatide doses with placebo over 72 weeks. The published average body weight change was:

Tirzepatide 5 mg

minus 15.0 percent

Tirzepatide 10 mg

minus 19.5 percent

Tirzepatide 15 mg

minus 20.9 percent

Placebo

minus 3.1 percent

All four figures were reported at 72 weeks. The three doses produced different averages within the one trial, which is why the dose is named beside each number rather than folded into a single result.

What these trials studied, and what they did not

Both trials studied the brand-name medications that contain semaglutide and tirzepatide. They did not study compounded semaglutide or compounded tirzepatide.

That distinction matters when a trial result is repeated next to a compounded product. Compounded semaglutide and compounded tirzepatide are not FDA-approved, and they were not the products used in STEP 1 or SURMOUNT-1. A figure from these trials describes the medicine that was actually tested in them, not a compounded preparation sold under the same molecule name.

Individual results vary. The percentages above are averages across the people enrolled in each trial, measured at the end of the follow-up period. They do not describe what any one person will experience, and they are not a prediction of someone else’s outcome.

Why this guide tracks cash prices

This guide is a record of what programs charge in cash for the compounded medications these trials made familiar. It does not repeat trial results as a price claim, and a price on this site is never presented as evidence of an outcome. To see the programs themselves, start with the buyer guide; to see how each charge was read and dated, read the methodology.