Semaglutide vs tirzepatide: which causes more muscle loss?
The trial numbers, why they’re easy to misread, and what decides how much muscle you keep.
Short answer
In separate trials, about 40% of the weight lost on semaglutide (STEP 1) and about 25% on tirzepatide (SURMOUNT-1) was lean mass. That suggests tirzepatide spares a larger share, but there’s no head-to-head body-scan comparison, and in kilos the gap is smaller. Strength training and protein matter more than which drug you take.
Not medical advice. GLP-1 medications are prescribed and supervised by a clinician. This page explains published trial results and how to track your own body; talk to your prescriber about your dose, protein and training.
What the trials found
| Semaglutide 2.4 mg (STEP 1) | Tirzepatide 15 mg (SURMOUNT-1) | |
|---|---|---|
| Average weight loss | About 15% at 68 weeks | About 21% at 72 weeks |
| Lean mass share of weight lost (DXA substudy) | About 40% | About 25% |
| Head-to-head body composition data | Not published in the trials we found | Not published in the trials we found |
On paper tirzepatide looks better for muscle: a smaller share of the loss was lean. But these are two different trials with different people, scan protocols and lengths. SURMOUNT-5 compared the two drugs directly and found more weight loss with tirzepatide (about 20% vs 14% at 72 weeks), but we haven’t seen a head-to-head DXA result.
Share vs kilos
A smaller share of a bigger loss can be a similar number of kilos. Losing 20 kg with 25% lean is 5 kg of lean mass; losing 15 kg with 40% lean is 6 kg. So the practical difference may be smaller than the percentages suggest, and your training and protein matter more than which drug you’re on.
What matters more than the drug
- Strength training two to three times a week.
- Enough protein, spread across meals.
- A pace your clinician is comfortable with.
- Tracking arms and shoulders against your waist so you see muscle loss early.
How to see it in your own body
Take matched front and side photos every two weeks and measure waist, arms and shoulders weekly. If your waist falls clearly faster than your arms, it’s mostly fat. ProgressLens reads exactly this, plus your photos, and shows a muscle-loss signal in each Progress Report; tell it you’re on a GLP-1 and the report is tailored to that.
FAQ
Frequently asked questions
Does tirzepatide cause less muscle loss than semaglutide?
In separate trials a smaller share of weight lost was lean mass on tirzepatide (about 25%) than on semaglutide (about 40%), but there’s no published head-to-head body-composition comparison we know of.
Does semaglutide cause more muscle loss than tirzepatide?
As a share of weight lost, the trial numbers point that way, but the trials differ, and in kilos the gap is smaller because tirzepatide causes more total weight loss.
Is Mounjaro better than Ozempic for keeping muscle?
Mounjaro is tirzepatide and Ozempic is semaglutide; see above. Your prescriber chooses the drug; training and protein are what you control.
Sources
- Wilding JPH et al. (2021). Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med 384:989–1002, with DXA substudy.
- Jastreboff AM et al. (2022). Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 387:205–216, with DXA substudy.
- Aronne LJ et al. (2025). Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med.
- Forbes GB. Body fat content influences the body composition response to nutrition and exercise. Ann N Y Acad Sci (2000).