Drug facts

Semaglutide and muscle loss

By ·Editor·Reviewed

Quick answer

Some of the weight lost on semaglutide is lean tissue, as with any large weight loss. Wegovy’s label says semaglutide lowers body weight with greater fat mass loss than lean mass loss. In an MRI analysis of the STEP UP trial, fat tissue fell by about 11 liters more than on placebo, while the drop in thigh muscle was about 1 liter and did not reach statistical significance. Exercise during weight loss helps limit the lean loss.

What the label says1

The label’s section on how the drug acts in the body opens with one sentence on this: semaglutide lowers body weight with greater fat mass loss than lean mass loss. It gives no percentage. Its trial results report body weight, as a percentage and in kilograms, plus the share of people who lost 5%, 10%, 15% or 20%. A scale cannot tell fat from muscle, so the detail comes from body-composition scans in separate studies. In the weight-loss trials, participants were also coached on a reduced-calorie diet and at least 150 minutes of activity a week.

What MRI scans found2

STEP UP was a 72-week trial comparing a 7.2 mg weekly dose, 2.4 mg and placebo in about 1,400 adults with obesity. A planned MRI substudy scanned 55 of them, 49 on semaglutide (mostly 7.2 mg) and 6 on placebo. Compared with placebo, the semaglutide group lost:

  • 11.1 liters more fat tissue, a clear difference.
  • 1.5 liters more visceral fat, the fat around the organs.
  • 1.7 liters more lean tissue and 1.1 liters more thigh muscle; neither difference was statistically significant.

Fat inside the thigh muscle also went down. Physical function held up: people did an average of 13 sit-to-stand repetitions in 30 seconds at the start and 15 at week 72. With only six people on placebo, the study is too small to rule out a real muscle loss, and the authors call for more research on muscle function.

What DXA scans found3

A DXA scan measures fat, lean mass and bone across the whole body. In a substudy of the SUSTAIN 8 trial, 178 adults with type 2 diabetes were scanned over a year of semaglutide 1 mg or another diabetes drug, canagliflozin. On semaglutide, fat mass fell by 3.4 kg and lean mass by 2.3 kg. Because fat fell faster, the share of the body that was lean went up by about 1.2 percentage points. The two drugs did not differ significantly, and with no placebo group the authors called the drug-specific effect uncertain.

How it compares with other weight loss4

A 2026 systematic review in the Annals of Internal Medicine pulled together 35 randomized trials of semaglutide and related drugs that measured body composition. Across them, a median of about 28% of the weight lost came from muscle-related measures, with a wide range from trial to trial. Losses above the reviewers’ benchmark also showed up in nearly half of the diet and lifestyle programs that led to weight loss, so this is not unique to the drugs. None of the trials in the review, which searched through February 2026, reported an objective test of physical function, so whether these losses matter for strength and daily life is still open.

Lean mass is not only muscle6

“Lean mass” on a body scan is a broader category than the muscle you use to climb stairs, and the two are easy to conflate in headlines. A 2026 review on semaglutide in postmenopausal women makes the point that a drop in lean body mass does not necessarily mean a clinically meaningful loss of skeletal muscle or of muscle function. It also flags older women as a group where muscle health deserves closer attention during treatment.

What helps hold on to muscle567

The best evidence here comes from diet trials, not semaglutide trials. A 2026 meta-analysis of 34 randomized trials found that adding exercise to calorie restriction prevented nearly half of the lean-mass loss, about 0.9 kg on average, with the biggest effects from mixed and strength training. The postmenopausal review suggests enough protein, adequate nutrients and resistance exercise may help, while noting that the evidence in people taking semaglutide is still indirect.

How much protein and what kind of training suits you is a question for your prescriber or a dietitian, particularly if you are older. Muscle also matters for what happens later: weight tends to come back after semaglutide stops, as STEP 4 showed, and what returns need not be the same tissue that was lost. Compounded semaglutide has not been studied for body composition; follow your pharmacy label and prescriber on dosing.

Questions

Does semaglutide cause muscle loss?
Some of the weight lost on semaglutide is lean tissue, as with any large weight loss. Wegovy's label says semaglutide causes greater fat loss than lean mass loss. In the STEP UP MRI substudy, fat tissue fell by about 11 liters more than placebo, while the extra thigh-muscle loss of about 1 liter was not statistically significant.
How much of the weight lost on semaglutide is muscle?
Estimates vary by study and scan type. On semaglutide 1 mg in SUSTAIN 8, people lost 3.4 kg of fat and 2.3 kg of lean mass. A review of 35 trials of semaglutide and related drugs found a median of about 28% of weight lost came from muscle-related measures.
Can exercise prevent muscle loss on semaglutide?
No semaglutide trial has tested it directly yet. In diet trials, adding exercise, especially strength training, prevented nearly half of the lean-mass loss. Reviews suggest resistance exercise and enough protein are reasonable, with the details set by a clinician.
Does Wegovy cause muscle loss?
Wegovy is semaglutide, so the same data apply. Its label states that semaglutide lowers body weight with greater fat mass loss than lean mass loss.
Is muscle loss on semaglutide a problem?
Not established either way. Scans show some lean-tissue loss, but a drop in lean mass does not necessarily mean a meaningful loss of muscle or strength. In the STEP UP MRI substudy, sit-to-stand performance held steady, but that was a small group, and a 2026 review of 35 earlier trials found none that tested physical function objectively.

Sources

  1. FDA prescribing information for WEGOVY (semaglutide), sections 12.2 and 14.2, revision 6/2026 DailyMed
  2. Hjelmesæth J et al., 'Effect of Semaglutide on Thigh Skeletal Muscle Volume, Fat Infiltration and Physical Function in People With Obesity: Secondary Analysis of STEP UP', Diabetes, Obesity and Metabolism 2026, online ahead of print (PMID 42764399), abstract
  3. McCrimmon RJ et al., 'Effects of once-weekly semaglutide vs once-daily canagliflozin on body composition in type 2 diabetes: a substudy of the SUSTAIN 8 randomised controlled clinical trial', Diabetologia 2020;63(3):473-485 (PMID 31897524), abstract
  4. Batsis JA et al., 'Effect of Incretin-Based and Nonpharmacologic Weight Loss on Body Composition: A Systematic Review', Annals of Internal Medicine 2026;179(7):996-1013 (PMID 41996180), abstract — liraglutide, semaglutide, tirzepatide and dulaglutide pooled
  5. Deller M et al., 'Effects of Calorie Restriction With and Without Strength, Endurance or Mixed Training on Fat-Free and Skeletal Muscle Mass in Overweight or Obese Individuals', Diabetes, Obesity and Metabolism 2026;28(8):6810-6823 (PMID 42144246), abstract — diet trials, not semaglutide
  6. Pereira NRM et al., 'Semaglutide in postmenopausal women: Clinical implications for weight loss, muscle health, and sleep-disordered breathing', Maturitas 2026;214:109129 (PMID 42772087), abstract — narrative review
  7. Rubino D et al., STEP 4, JAMA 2021;325(14):1414-1425 (PMID 33755728), abstract