Does tirzepatide cause more muscle loss than semaglutide?

Not based on the substudy data. Tirzepatide's SURMOUNT-1 substudy showed a lower share of lean mass loss (about 25% of total weight lost) than semaglutide's STEP 1 substudy (around 40%), and tirzepatide's ratio matched its own placebo group almost exactly. Total weight loss tends to be larger with tirzepatide, though, so the absolute amount of lean mass lost can still be bigger even at a lower percentage.

Will lost muscle come back if I stop taking a GLP-1?

This isn't well studied yet. Muscle generally responds to how much you use it and how much protein you eat, regardless of what caused the original loss, so resistance training and adequate protein after stopping the medication would be expected to help rebuild it. There's no dedicated long-term trial confirming that timeline in people coming off GLP-1s specifically.

Are older adults at higher risk of muscle loss on GLP-1 medications?

Age-related muscle loss (sarcopenia) is already a concern independent of any medication, so a large, fast weight loss on top of that is worth discussing with your care team if you're older or already have low muscle mass. This is exactly the kind of individual risk factor that general trial averages don't capture well.

Is it worth getting a DEXA or InBody scan to check my muscle mass?

It can give you a real baseline instead of a guess, especially if you plan to track change over months. It's not required, though. Simpler signals, like whether your strength on familiar exercises is holding up, tend to track reasonably well with what a scan would tell you.

Does walking count as exercise for preserving muscle, or do I need to lift weights?

Walking is good for plenty of things, but the research on preserving muscle specifically points to resistance training, the kind that loads a muscle against something, bands, weights, or your own bodyweight. Walking alone hasn't shown the same muscle-preserving signal in the literature.

Is muscle loss a reason to avoid GLP-1 medications altogether?

That's a decision to make with your prescriber, weighing your own health risks and goals, not something a general research summary can answer for you. What the research does suggest is that the muscle loss risk is manageable with protein and resistance training, not a reason on its own to rule the medication out.

Does Ozempic Cause Muscle Loss? Here's What the Research Actually Shows

Quick answer: Yes, some of the weight lost on GLP-1 medications is muscle. Semaglutide's trials show a higher share of lean mass loss than typical weight loss, while tirzepatide's trials show the same ratio as placebo, meaning the drug itself isn't the obvious cause there. A 2026 study even found muscle strength and relative mass improved despite the drop in absolute terms. Resistance training is a reasonable precaution, though the trial proving it works in GLP-1 users specifically hasn't finished yet.

If you've spent any time in GLP-1 forums or watched more than one TikTok about Ozempic, you've heard it: you're not just losing fat, you're losing muscle. People toss it around like settled fact, some as a warning, some like it's just common knowledge by now, and almost nobody can say where the number comes from.

Let's go find out.

What the Trials Found

Two big body composition substudies exist, one for semaglutide and one for tirzepatide, and they tell slightly different stories.

In the STEP 1 trial, people on semaglutide lost 15% of their body weight over 68 weeks, compared to 3.6% on placebo. Fat mass dropped 19.3%. Lean body mass dropped too, by 9.7%. Do the math on those numbers against the total weight lost, and lean mass comes out to somewhere around 40% of it. That's higher than the roughly 25% that shows up in most weight loss research generally, a ratio researchers sometimes call the "quarter fat-free mass rule."

Tirzepatide tells a different story. In the SURMOUNT-1 substudy, people lost 21.3% of body weight, with fat mass down 33.9% and lean mass down 10.9%. Worked out as a share of total weight lost, that's about 75% fat and 25% lean. Here's the detail that matters: the placebo group in that same study lost weight in almost the exact same ratio. Whatever caused the lean mass loss, it wasn't unique to the drug.

So the honest answer isn't "yes" or "no." It's "it depends which drug, and even then, some of what you're seeing might just be what happens when anyone loses a lot of weight fast."

Bar chart comparing the share of weight lost that was lean mass: semaglutide in STEP 1 at roughly 40%, above typical, while tirzepatide, tirzepatide placebo, and typical weight loss all sit around 25%

Semaglutide figure is a calculated estimate from the STEP 1 body composition substudy's published fat and lean mass changes, not a number stated directly in the trial itself. Tirzepatide and placebo figures are as reported in the SURMOUNT-1 substudy. 'Typical weight loss' reflects the commonly cited ~25% lean-mass share seen across non-drug weight loss research.

Is "Lean Mass" the Same Thing as "Muscle"?

Not exactly, and this is where a lot of the online panic gets sloppy.

"Lean body mass," the number that shows up in these DEXA scans, includes muscle, but it also includes organs, bone, and water. A 2024 paper in Advances in Nutrition dug into exactly this kind of terminology confusion, arguing that "lean body mass" is a broader category than muscle alone and gets used loosely in ways that blur the distinction. When a headline says "40% of the weight lost was lean mass," that's not automatically the same claim as "40% of the weight lost was muscle."

A study published in Cell Reports Medicine in March 2026 dug into this more directly, using mice and a small proof-of-concept human trial. The finding that got attention: liver mass dropped more than muscle mass did. And while absolute muscle mass and strength went down slightly, relative muscle mass and running performance got better, not worse. The researchers didn't find evidence that the muscle loss was disproportionate or medically concerning.

Worth being honest about the limits here too. The human portion of that study was small, and the animal work only used male mice. It's promising, but it's early.

Why This Happens With Any Fast Weight Loss

Losing weight quickly, by any method, tends to pull some muscle along with the fat. This isn't a GLP-1-specific side effect so much as a basic feature of how the body responds to a large, sustained calorie deficit. The "quarter rule" mentioned above (roughly a quarter of weight lost being lean mass) shows up across plenty of non-drug weight loss studies too.

What's different with GLP-1s is the speed and size of the deficit. Appetite drops hard and fast, calorie intake falls without much conscious effort, and if protein intake falls along with everything else, there's less raw material available to protect muscle while the deficit is doing its work.

Speed is the real culprit here, not the molecule.

Can You Prevent the Muscle Loss?

Protein and resistance training both come up constantly in this conversation, and both make sense on paper. The evidence behind each one is thinner than the confident Instagram infographics let on.

A 2024 review in Diabetes Care laid out the case for resistance training as a companion to GLP-1 therapy, arguing it should help preserve muscle during incretin-driven weight loss. But the authors were upfront that this is a proposal built on general exercise science, not a finding from a completed trial in GLP-1 users specifically. They said as much themselves: future studies are needed to test it in this population. That trial is now running. It's called LEAN-PREP, a randomized controlled trial testing resistance exercise and protein intake in people starting semaglutide or tirzepatide, and as of this writing it's still recruiting.

Until that trial reads out, the honest label for resistance training is "reasonable bet," not "proven fix."

Protein is on firmer ground, if less dramatic than the headlines suggest. General nutrition research on preserving muscle during weight loss points to somewhere around 1.2 to 1.6 grams of protein per kilogram of body weight daily, though individual needs vary with age, activity level, and how much weight you're losing. A dietitian or your care team can help you land on a number that fits your situation.

The Bottom Line

So what should you do with all this? Don't treat "muscle loss" as a reason to avoid GLP-1 medications outright, the trial data doesn't support that leap, especially on tirzepatide, where the ratio matched placebo exactly. Do treat protein as the one lever with solid evidence behind it right now, and don't wait for LEAN-PREP to finish before you start lifting something heavy twice a week. There's basically no downside to starting early. If you're older, already dealing with low muscle mass, or losing weight unusually fast, that's a conversation for your care team, not something a trial average can settle for you. The rest of this is a genuinely moving target, worth checking back on once LEAN-PREP and any follow-up human work on the Cell Reports Medicine findings report results.

Protein is the piece you have real control over, so it's worth knowing where you stand day to day instead of guessing. That's the whole idea behind SNAQ: snap a photo of a meal and get a protein estimate back, no math required. Download the SNAQ app to make that easier, or read our full guide to tracking meals and appetite changes on GLP-1 for the complete walkthrough.

References

Wilding JPH, Batterham RL, Calanna S, et al. Impact of Semaglutide on Body Composition in Adults With Overweight or Obesity: Exploratory Analysis of the STEP 1 Study. Journal of the Endocrine Society, 2021;5(Suppl 1):A16-A17. https://doi.org/10.1210/jendso/bvab048.030
Look M, et al. Body Composition Changes During Weight Reduction With Tirzepatide in the SURMOUNT-1 Study of Adults With Obesity or Overweight. Diabetes, Obesity and Metabolism, 2025. https://doi.org/10.1111/dom.16275
Langer HT, Gilmore NK, Hayden CMT, et al. Weight Loss With GLP-1 Medicines Does Not Result in a Disproportionate Loss of Muscle Mass or Function in Obese Mice and Humans. Cell Reports Medicine, 2026;7(3):102665. https://doi.org/10.1016/j.xcrm.2026.102665
Heymsfield SB, Brown J, Ramirez S, Prado CM, Tinsley GM, Gonzalez MC. Are Lean Body Mass and Fat-Free Mass the Same or Different Body Components? A Critical Perspective. Advances in Nutrition, 2024;15(12):100335. https://doi.org/10.1016/j.advnut.2024.100335
Locatelli JC, Costa JG, Haynes A, Naylor LH, Fegan PG, Yeap BB, Green DJ. Incretin-Based Weight Loss Pharmacotherapy: Can Resistance Exercise Optimize Changes in Body Composition? Diabetes Care, 2024;47(10):1718-1730. https://doi.org/10.2337/dci23-0100
LEAN Mass Preservation With Resistance Exercise and Protein During Semaglutide and Tirzepatide Therapy (LEAN-PREP Study). ClinicalTrials.gov identifier NCT06885736. (Ongoing trial, referenced for context; not yet reporting results.)

Does Ozempic Cause Muscle Loss? Here's What the Research Actually Shows

What the Trials Found

Is "Lean Mass" the Same Thing as "Muscle"?

Why This Happens With Any Fast Weight Loss

Can You Prevent the Muscle Loss?

The Bottom Line

References

Frequently asked questions

About the author

Tracking food on a GLP-1?

What the Trials Found

Is "Lean Mass" the Same Thing as "Muscle"?

Why This Happens With Any Fast Weight Loss

Can You Prevent the Muscle Loss?

The Bottom Line

References

Frequently asked questions

About the author

Tracking food on a GLP-1?

More from the SNAQ blog.