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

What the two pivotal trials measured and when, how early the curves separated from placebo, the final totals, and why none of it is a forecast for you.

3 min readUpdated September 4, 2026By FormBlends editorial team4 sourcesEducational, not medical advice

SURMOUNT-1: weight change by arm at 72 weeks

From the data
SURMOUNT-1: Tirzepatide 15 mg changed body weight -20.9 percent at 72 weeks versus -3.1 percent on placebo.Horizontal bars for each arm of SURMOUNT-1. Tirzepatide 5 mg: -15 percent; Tirzepatide 10 mg: -19.5 percent; Tirzepatide 15 mg: -20.9 percent; Placebo: -3.1 percent.Percent change in body weight (loss shown as bar length)0%-5%-10%-15%-20%-25%Tirzepatide 5 mg-15%-11.9 points versus placeboTirzepatide 10 mg-19.5%-16.4 points versus placeboTirzepatide 15 mg-20.9%-17.8 points versus placeboPlacebo-3.1%
Mean percent change in body weight from baseline, 2,539 randomised. Difference labels subtract the placebo mean from each active arm's mean. Whiskers are the 95 percent confidence intervals from the abstract. Source: PubMed 35658024. Source: N Engl J Med 2022;387(3):205-216, PubMed 35658024.

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

STEP 1

Semaglutide 2.4 mg weekly vs placebo, 68 weeks

Participants
1,961 adults, BMI 30 or higher (or 27 with a weight-related condition), no diabetes
Mean baseline weight
105.3 kg
Escalation
16 weeks to 2.4 mg
Mean change at 68 weeks
-14.9% vs -2.4% placebo
Difference
-12.4 percentage points (95% CI -13.4 to -11.5)
Reached 5% / 10% / 15%
86.4% / 69.1% / 50.5% vs 31.5% / 12.0% / 4.9%

Wilding et al., NEJM 2021, PubMed 33567185. Treatment-policy estimand.

SURMOUNT-1

Tirzepatide 5, 10, 15 mg weekly vs placebo, 72 weeks

Participants
2,539 adults, BMI 30 or higher (or 27 with a weight-related condition), no diabetes
Mean baseline weight
104.8 kg
Escalation
4 weeks per 2.5 mg step to the assigned dose
Mean change at 72 weeks
-15.0% / -19.5% / -20.9% vs -3.1% placebo
Reached 5%
85% / 89% / 91% vs 35%
Stopped for adverse events
4.3% / 7.1% / 6.2% vs 2.6%

Jastreboff et al., NEJM 2022, PubMed 35658024. Treatment-regimen estimand.

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.

Questions people ask

How much weight do people lose in the first month?

The papers do not tabulate a month-one figure. Figure 1 in each shows the treatment curve already below placebo at week 4, the first visit, with a slope that continues through escalation. The published totals are at 68 weeks (semaglutide) and 72 weeks (tirzepatide). Any specific first-month percentage you see quoted is a reading from a graph, not a reported result.

Did everyone in the trials lose weight?

No. In STEP 1, 86.4% of the semaglutide group lost at least 5% of body weight, so about one in seven did not. In SURMOUNT-1, 85 to 91% reached 5% depending on dose. The averages are pulled from a wide spread of individual results.

Sources (4)

  1. Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med 2021;384:989-1002. PubMed 33567185 Accessed September 4, 2026.
  2. Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 2022;387:205-216. PubMed 35658024 Accessed September 4, 2026.
  3. Wegovy (semaglutide) prescribing information, section 14, Novo Nordisk, DailyMed set id ee06186f-2aa3-4990-a760-757579d8f77b Accessed September 4, 2026.
  4. Zepbound (tirzepatide) prescribing information, section 14, Eli Lilly, DailyMed set id 487cd7e7-434c-4925-99fa-aa80b1cc776b Accessed September 4, 2026.

Canonical URL: https://formblendsglp1.com/weeks/when-weight-change-starts. Written by the FormBlends editorial team. This page is educational and is not medical advice; see the medical disclaimer.