Evidence library

How much weight do GLP-1s actually take off?

The weight-loss figures are real and large, but the average hides a lot: your result depends on which drug, whether you have diabetes, and the support around the prescription. Here is what the trials measured, and what the numbers leave out.

How to read these grades

Evidence strength

  • Strong evidence (does / reduces / improves)
  • Moderate evidence (probably / likely)
  • Early / weak evidence (may / might)

Tested in GLP-1 users?

  • Tested in GLP-1 users
  • Not tested in GLP-1 users
  • Trial underway

In general obesity, semaglutide takes off about 15% of body weight

Strong evidence Tested in GLP-1 users

In adults with obesity but not diabetes, once-weekly semaglutide 2.4 mg reduced body weight by about 15% over 68 weeks, roughly two to three times what older weight-loss drugs achieved. That figure is a trial average, not a personal guarantee, and the trial included structured support.

What it rests on

  • STEP-1 primary 1

    The pivotal trial of semaglutide 2.4 mg in adults with obesity but not diabetes. Over 68 weeks people lost about 15% of body weight versus about 2% on placebo, two to three times what older weight-loss drugs achieved.

    Semaglutide 2.4 mg, obesity without diabetes

  • Mozaffarian 2025 advisory 2

    A joint advisory from four medical societies summarizing current best practice for supporting people on GLP-1s. It is an expert consensus document, not a trial, and is the standard reference for what the field recommends and where evidence is still missing.

    Four-society expert consensus

With type 2 diabetes, the weight loss is meaningfully smaller

Strong evidence Tested in GLP-1 users

People with type 2 diabetes lose less weight on the same drug and dose, about 10% rather than the roughly 15% seen without diabetes. The drug still works, and it lowers blood sugar; the weight effect is simply attenuated.

What it rests on

  • Davies 2021 (STEP 2) 3

    STEP 2 tested the same semaglutide 2.4 mg dose in people who also had type 2 diabetes. They lost about 9.6% versus 3.4% on placebo, meaningfully less than the roughly 15% seen without diabetes, alongside a 1.6-point drop in HbA1c.

    Semaglutide 2.4 mg, type 2 diabetes

  • Mozaffarian 2025 advisory 2

    A joint advisory from four medical societies summarizing current best practice for supporting people on GLP-1s. It is an expert consensus document, not a trial, and is the standard reference for what the field recommends and where evidence is still missing.

    Four-society expert consensus

Those trial numbers came with intensive support, not the drug alone

Strong evidence Tested in GLP-1 users

The headline figures were achieved alongside structured lifestyle support: frequent counseling and dietary guidance layered on top of the drug. In the trial that added intensive behavioral therapy, even the placebo group lost 5.7%, more than double the placebo loss in lighter-counseling trials.

What it rests on

  • Wadden 2021 (STEP 3) 4

    STEP 3 paired the drug with intensive behavioral therapy and a starting low-calorie diet. That group lost about 16%, but the placebo group getting the same intensive support still lost 5.7%, more than double the placebo loss in trials with lighter counseling.

    Semaglutide 2.4 mg + intensive therapy

  • Mozaffarian 2025 advisory 2

    A joint advisory from four medical societies summarizing current best practice for supporting people on GLP-1s. It is an expert consensus document, not a trial, and is the standard reference for what the field recommends and where evidence is still missing.

    Four-society expert consensus

The weight loss lasts as long as you stay on the drug, out to two years

Strong evidence Tested in GLP-1 users

On continued treatment the weight loss is durable: it plateaus around a year and holds through two years rather than creeping back. "Durable" here means while you are still taking it; stopping brings regain.

What it rests on

  • Garvey 2022 (STEP 5) 5

    STEP 5 followed people for two years. Weight loss reached about 15% by week 52 and held at 15% at week 104, plateauing around a year rather than creeping back, versus about 2.6% on placebo.

    Semaglutide 2.4 mg, 2 years

  • Mozaffarian 2025 advisory 2

    A joint advisory from four medical societies summarizing current best practice for supporting people on GLP-1s. It is an expert consensus document, not a trial, and is the standard reference for what the field recommends and where evidence is still missing.

    Four-society expert consensus

In real-world use, tirzepatide out-loses semaglutide

Moderate evidence Tested in GLP-1 users

In routine practice, people on tirzepatide probably lose more than those on semaglutide, a gap of roughly 15% versus 8% at a year. Because this is a matched real-world cohort rather than a randomized trial, other differences between the groups could contribute.

What it rests on

  • Rodriguez 2024 6

    A real-world study matched people starting tirzepatide against people starting semaglutide. At 12 months the tirzepatide group had lost more (about 15% vs 8%). Because it is not randomized, other differences between the groups could contribute to the gap.

    Real-world matched cohort

  • Mozaffarian 2025 advisory 2

    A joint advisory from four medical societies summarizing current best practice for supporting people on GLP-1s. It is an expert consensus document, not a trial, and is the standard reference for what the field recommends and where evidence is still missing.

    Four-society expert consensus

Averages hide wide variation, and not everyone responds

Moderate evidence Tested in GLP-1 users

The average figures mask a wide spread: most people lose at least 5%, far fewer reach 15 to 20%, and a minority respond little. "About 15%" is a mean across trials, not a result to expect for any one person.

What it rests on

  • STEP-1 primary 1

    The pivotal trial of semaglutide 2.4 mg in adults with obesity but not diabetes. Over 68 weeks people lost about 15% of body weight versus about 2% on placebo, two to three times what older weight-loss drugs achieved.

    Semaglutide 2.4 mg, obesity without diabetes

  • Wadden 2021 (STEP 3) 4

    STEP 3 paired the drug with intensive behavioral therapy and a starting low-calorie diet. That group lost about 16%, but the placebo group getting the same intensive support still lost 5.7%, more than double the placebo loss in trials with lighter counseling.

    Semaglutide 2.4 mg + intensive therapy

  • Garvey 2022 (STEP 5) 5

    STEP 5 followed people for two years. Weight loss reached about 15% by week 52 and held at 15% at week 104, plateauing around a year rather than creeping back, versus about 2.6% on placebo.

    Semaglutide 2.4 mg, 2 years

  • Davies 2021 (STEP 2) 3

    STEP 2 tested the same semaglutide 2.4 mg dose in people who also had type 2 diabetes. They lost about 9.6% versus 3.4% on placebo, meaningfully less than the roughly 15% seen without diabetes, alongside a 1.6-point drop in HbA1c.

    Semaglutide 2.4 mg, type 2 diabetes

Sources

  1. 1. Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002. doi:10.1056/nejmoa2032183
  2. 2. Mozaffarian D, Agarwal M, Aggarwal M, et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. The American Journal of Clinical Nutrition. 2025;122(1):344-367. doi:10.1016/j.ajcnut.2025.04.023
  3. 3. Davies M, Færch L, Jeppesen OK, et al. Semaglutide 2·4 mg once a week in adults with overweight or obesity, and type 2 diabetes (STEP 2): a randomised, double-blind, double-dummy, placebo-controlled, phase 3 trial. The Lancet. 2021;397(10278):971-984. doi:10.1016/s0140-6736(21)00213-0
  4. 4. Wadden TA, Bailey TS, Billings LK, et al. Effect of Subcutaneous Semaglutide vs Placebo as an Adjunct to Intensive Behavioral Therapy on Body Weight in Adults With Overweight or Obesity. JAMA. 2021;325(14):1403. doi:10.1001/jama.2021.1831
  5. 5. Garvey WT, Batterham RL, Bhatta M, et al. Two-year effects of semaglutide in adults with overweight or obesity: the STEP 5 trial. Nat Med. 2022;28(10):2083-2091. doi:10.1038/s41591-022-02026-4
  6. 6. Rodriguez PJ, Goodwin Cartwright BM, Gratzl S, et al. Semaglutide vs Tirzepatide for Weight Loss in Adults With Overweight or Obesity. JAMA Intern Med. 2024;184(9):1056. doi:10.1001/jamainternmed.2024.2525