When weight change typically starts: the STEP 1 and SURMOUNT-1 curves

Two trials produced the numbers everyone quotes. This page is what they measured, when, and how to read the curves as a beginner. The network's FormBlends Research site has full digests of both, plus the rest of the STEP and SURMOUNT programs.

The two trials in one table

When the curves separate

Both papers plot mean percent change from baseline at each visit (Figure 1 in each). Both trials weighed people at week 4 for the first time after baseline, and at that visit the treatment curves already sit below the placebo curve. From there the slope continues through the escalation weeks and well beyond. In STEP 1 the curve is still falling past week 52 and flattens toward week 60. In SURMOUNT-1 the three tirzepatide curves are still falling at week 72 for the higher doses.

Two honest cautions about reading those figures. First, the papers do not tabulate the intermediate points; any "week 12 average" you see quoted is somebody's reading of a graph. Second, the participants were escalating during the early weeks, so the early slope reflects rising doses, not the maintenance dose you may end up on. This site does not print intermediate percentages for those reasons. The weight-loss projection tool on the calculator site applies the published endpoint averages to your weight and labels them as what they are.

What "average" hides

A mean of 14.9% is made of people who lost 30% and people who lost 3%. The responder rates are the more useful numbers for a beginner: roughly one in seven people on semaglutide 2.4 mg did not reach 5% at 68 weeks, and about half reached 15%. Roughly one in ten on tirzepatide did not reach 5%. Where you land is not knowable in advance, and the first twelve weeks are a poor predictor because escalation is still under way. The weeks 9 to 12 page covers how to think about early numbers.

What was around the drug

Every participant in both trials also received a reduced-calorie diet (a 500 kcal per day deficit) and a physical activity target (150 minutes per week) with regular counselling. The placebo groups lost 2.4% and 3.1% on that programme alone. The labels carry the same framing: the drug is indicated in combination with a reduced-calorie diet and increased physical activity. The Weight Loss on GLP-1s site covers the food and training side.

Who was not in the trials

Both excluded people with diabetes (separate trials, STEP 2 and SURMOUNT-2, covered those), people with a history of pancreatitis, and the groups in the label contraindications. Both enrolled adults with BMI 30 or higher, or 27 or higher with at least one weight-related condition. If you are outside that population the averages are less informative still.

What happens after the trials stopped

That is a separate and important question, with its own data, on the myths page under "the weight comes straight back".

Compounded products

Both trials used the brand products made by Novo Nordisk and Lilly. No comparable trial exists for compounded semaglutide or tirzepatide, which are not FDA approved and are not interchangeable with the brand products. Applying these curves to a compounded product assumes equivalence that has not been tested. FormBlends, which runs this site, sells compounded products and discloses that on the brand versus compounded page.