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What the GLP-1 weight loss trials measured.

Semaglutide and tirzepatide were each tested in large trials. Here is who took part, what they took, for how long, what was measured and what was found, with a source for every figure.

Looking for a weight loss simulator? This page has none on purpose. A simulator applies a trial average to your weight, and a trial average cannot tell you what your own weight will do.

Results vary. Every figure below is an average for one group in one trial, and results within each group ranged widely. None of it is a forecast for you. Only a prescriber can advise you about a medication and what to expect from it.

The medicines in these trials.

SemaglutideSold as Wegovy and Ozempic
Semaglutide acts like GLP-1, a hormone the gut releases after eating that plays a part in appetite and blood sugar. In the trials below it was a once-weekly injection.
TirzepatideSold as Zepbound and Mounjaro
Tirzepatide acts on the receptors for two of these gut hormones, GIP and GLP-1. In the trials below it was a once-weekly injection.

In the United States, Wegovy and Zepbound are approved for chronic weight management, and Ozempic and Mounjaro for type 2 diabetes. Approvals differ between countries. All four trials below enrolled adults without diabetes.

What each trial measured.

  1. STEP 1

    Who took part
    1,961 adults with a BMI of 30 or more, or 27 or more with at least one weight-related condition, and without diabetes.
    What they took
    Semaglutide 2.4 mg or a placebo once a week, alongside diet and activity counseling.
    How long
    68 weeks.
    What was measured
    The average percentage change in body weight from the start of the trial.
    What it found
    Body weight fell by an average of 14.9% on semaglutide and 2.4% on placebo.
    Source
    Wilding et al., New England Journal of Medicine, 2021 (opens in a new tab)

  2. SURMOUNT-1

    Who took part
    2,539 adults with a BMI of 30 or more, or 27 or more with at least one weight-related condition, and without diabetes.
    What they took
    Tirzepatide 5 mg, 10 mg or 15 mg, or a placebo, once a week, alongside diet and activity counseling. The dose was raised in steps over the first 20 weeks.
    How long
    72 weeks.
    What was measured
    The average percentage change in body weight from the start of the trial.
    What it found
    Body weight fell by an average of 15.0% on 5 mg, 19.5% on 10 mg and 20.9% on 15 mg, compared with 3.1% on placebo.
    Source
    Jastreboff et al., New England Journal of Medicine, 2022 (opens in a new tab)

  3. STEP 1 extension

    Who took part
    327 people who had completed the 68 weeks of STEP 1, followed for one more year.
    What they took
    No treatment. At week 68, semaglutide, placebo and the counseling all stopped.
    How long
    One year after stopping, to week 120.
    What was measured
    How body weight changed after treatment stopped. The authors describe these analyses as exploratory.
    What it found
    People on semaglutide had lost an average of 17.3% by week 68. By week 120 they had regained 11.6 percentage points, about two thirds of that loss, for a net loss of 5.6%. The placebo group ended 0.1% below where it started.
    Source
    Wilding et al., Diabetes, Obesity and Metabolism, 2022 (opens in a new tab)

  4. SURMOUNT-4

    Who took part
    783 adults with a BMI of 30 or more, or 27 or more with at least one weight-related condition, and without diabetes. The 670 who completed the first 36 weeks went on to the second part.
    What they took
    For 36 weeks, everyone took tirzepatide once a week at the highest dose they tolerated, 10 mg or 15 mg. Then half kept taking it and half were switched to a placebo, without knowing which. Both parts included diet and activity counseling.
    How long
    88 weeks: 36 weeks on tirzepatide, then 52 weeks on tirzepatide or placebo.
    What was measured
    The average percentage change in body weight from week 36 to week 88.
    What it found
    Among those 670, body weight fell by an average of 20.9% over the first 36 weeks. From week 36 to week 88, it fell a further 5.5% in the group that kept taking tirzepatide and rose 14.0% in the group switched to placebo.
    Source
    Aronne et al., JAMA, 2024 (opens in a new tab)

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This page summarizes published clinical trials for general information. It is not a prediction of your results and not medical advice. Results vary from person to person. Only your prescriber can advise you about a medication, a dose or what to expect.