Why GLP-1 weight loss can slow down over time

Weight loss is a curve, not a straight line

If you picture weight loss on a GLP-1 as a steady downward line, the trial data tells a more nuanced story: the change is real, but it unfolds over many months — fastest early on, then gentler.

The pivotal semaglutide trial, STEP 1, followed adults through 68 weeks of the once-weekly injection. Over that period, the mean change in body weight was −14.9% with semaglutide versus −2.4% with placebo — a sustained, clinically relevant reduction when paired with lifestyle changes.¹

The tirzepatide trial, SURMOUNT-1, ran over 72 weeks across three doses. The mean change at week 72 was −15.0% (5 mg), −19.5% (10 mg), and −20.9% (15 mg), versus −3.1% with placebo.³ In both trials, those headline numbers are the end point of a long, gradual curve — not a fixed weekly rate.

Why the curve flattens: the plateau is part of the pattern

Across these trials, weight loss didn’t keep dropping forever — it slowed and leveled off into what researchers call a plateau. In the two-year STEP 5 trial, semaglutide produced a substantial initial reduction that plateaued after approximately week 60 and was then maintained for the rest of the study.²

Researchers have measured how predictable this is. In a post-hoc analysis of the SURMOUNT trials, a plateau was defined as under 5% weight change over a 12-week interval and all later intervals; by that definition, most participants reached a plateau by week 72, and in SURMOUNT-1 roughly 88–90% had plateaued across BMI categories.⁴

The timing varied. In SURMOUNT-1, the median time to a plateau ranged from about 24.3 weeks (overweight category) to 36.1 weeks (class III obesity), and higher doses, younger age, and female sex were each associated with reaching a plateau later.⁴ A slowing curve is the typical trajectory these studies describe; where it flattens differs across individuals.

Leveling off isn’t the same as losing the progress

It’s worth separating two ideas: weight loss slowing down, and weight loss reversing. In STEP 5, mean weight loss was about −15.6% at week 52 and −15.2% at week 104 — figures the authors said suggest minimal weight regain over 104 weeks.² The curve flattened, but the loss was largely held.

What changed the picture was stopping the medicine. In the STEP 1 extension, after participants came off semaglutide they regained 11.6 percentage points of their lost weight by week 120, versus 1.9 points with placebo, and cardiometabolic improvements reverted toward baseline.⁵

So the well-supported picture is a curve that drops quickly at first, slows over many months, and tends to settle into a plateau that holds while treatment continues. A tracker can help you see your own version of that curve over time, but it isn’t a medical provider — questions about your individual trajectory, your dose, or any change to treatment belong with the clinician who prescribed it.