Muscle loss is one of the most common worries people have before starting Mounjaro or Wegovy. And it's a fair one, given how effective both treatments are at supporting weight loss.
So, do Mounjaro and Wegovy cause muscle loss, and if so, how much? The short answer is that it's the weight loss they cause, not the medicines acting on muscle directly, that can affect how much you keep.
Despite what's often quoted online, the evidence is more reassuring than you might think. But it does depend on how you approach the rest of your lifestyle during treatment.
This guide looks at what clinical trial data actually shows, who's most at risk of losing more than average, how Wegovy and Ozempic compare, and what you can do to protect your muscle while losing weight.
It's the weight loss, not the medicine, that affects muscle. And that's something you can influence.
Does Mounjaro cause muscle loss?
Not directly. Mounjaro doesn't act on muscle tissue. What it causes is significant weight loss, and it's the weight loss itself that can reduce muscle mass, the same way any substantial weight loss can.
In the SURMOUNT-1 trial, the main study behind Mounjaro's approval, researchers measured body composition directly using DXA body scans, rather than just tracking weight on the scales.
Of the weight lost with tirzepatide, around three quarters was fat and about a quarter was lean mass. That split was almost identical to the placebo group, who lost weight through diet and lifestyle changes alone, even though they lost far less overall.1
That similarity is telling. It suggests muscle loss tracks with weight loss, rather than Mounjaro acting on muscle independently.
Why does Mounjaro, or any GLP-1 medication, cause muscle loss?
Mounjaro and Wegovy don't target muscle directly. They reduce your appetite and slow how quickly your stomach empties, so you eat less and end up in a calorie deficit.2
When your body is in a calorie deficit for a while, it draws energy from stored fat. But it also breaks down some muscle for fuel. That happens with any method of weight loss, whether it's diet, exercise or medication.
How much muscle you lose can be influenced by how quickly you lose weight, how much protein you eat and how active you are. Researchers who reviewed the evidence on lean mass during GLP-1 treatment found there's no fixed rule for how much any one person will lose.
The proportion varies with total energy intake, diet, sex, starting body composition and activity levels, not just the dose.3 That's why two people on the same dose of Mounjaro can end up with quite different results.
Does Wegovy cause muscle loss?
The same is true of Wegovy, for the same reason. It's the weight loss it causes, not the medicine acting on muscle, that accounts for any drop in lean mass.
Wegovy contains semaglutide. In a dedicated body composition study of people on semaglutide 2.4mg, lean mass fell in the first seven months of treatment, then levelled off.
Meanwhile, handgrip strength actually improved by around 4.5kg by the twelve-month mark.4 That's a measure of how well your muscles work, not just how big they are.
Muscle size and muscle strength aren't the same thing. Strength can improve even while lean mass dips.
The same study found the proportion of people with sarcopenic obesity, a mix of high fat mass and low muscle function, fell from 49% at the start of treatment to 33% after a year.4
In other words, losing some lean mass early on doesn't necessarily mean your muscles are getting weaker overall. Especially once your body adjusts, and strength training and protein come into the picture.
Does Ozempic cause muscle loss?
This is a common question, but it needs some clarification. Ozempic contains the same active ingredient as Wegovy, semaglutide. But in the UK, it's licensed only for managing type 2 diabetes, not for weight loss.5 Wegovy is the semaglutide medicine licensed specifically for weight management.
Because they share the same active ingredient, the body composition evidence for semaglutide broadly applies to both. That said, diabetes doses are usually lower than weight loss doses.
As with Mounjaro and Wegovy, any muscle loss linked to Ozempic would come from the weight loss, not a direct effect on muscle. If your goal is weight management, Wegovy is the appropriately licensed option, and a GP or prescriber can advise on which treatment fits your circumstances.
Who is most at risk of losing more muscle?
Averages don't apply evenly to everyone. A few groups tend to lose a bigger share of muscle than the figures above suggest:
- Older adults - muscle mass naturally declines with age, so there's less to spare and less capacity to rebuild it quickly.
- Women - women usually start with less muscle than men, so the same amount of muscle loss is a bigger proportion of what they have.
- People eating very little protein - appetite suppression makes this more likely, since eating less overall usually means eating less protein too.
- People losing weight very quickly - faster weight loss is consistently linked to more of that weight coming from lean mass, whatever the cause.
A study presented at the Endocrine Society's 2025 annual meeting compared people on semaglutide with people following a diet and lifestyle programme alone. Overall, the proportion of weight lost as lean mass was similar between the two groups.
But within the semaglutide group, being older, being female or eating less protein were each linked to more muscle loss. And losing more muscle was, in turn, linked to smaller improvements in blood sugar control.6
None of these risk factors are fixed in stone. Protein intake and resistance training are both things you can change, which is where the practical advice below comes in.
How much muscle do you actually lose?
You may have seen claims online that up to 40% of the weight lost on GLP-1 medicines is muscle. The controlled trial data doesn't support a figure that high. As covered above, the SURMOUNT-1 body composition substudy found lean mass made up around a quarter of the weight lost on tirzepatide, in line with placebo.1
Real-world data tells a slightly more complicated story. An early analysis of routine health records from over 670,000 people on semaglutide or tirzepatide, which hasn't yet been fully peer reviewed, found that a meaningful minority lost noticeably more lean mass than trial participants.7
This was especially true of people who had joint pain or reduced exercise tolerance before starting treatment. In that analysis, how much exercise capacity someone had to begin with was a stronger predictor of muscle loss than the medicine or the dose.7
That detail matters. Clinical trials include structured advice on protein and activity as standard, which real-world treatment doesn't always include to the same degree. It's one of the clearest signs that staying active during treatment is what decides how much muscle you keep.
The biggest predictor of muscle loss in real-world data wasn't the medicine. It was how active people were to begin with.
Does it actually matter if you lose some muscle?
Muscle isn't just about how your body looks. It's metabolically active, meaning it burns energy even at rest, and it plays a part in blood sugar control, bone health and everyday tasks like climbing stairs or carrying shopping.
Losing a meaningful amount of it can lower your resting metabolic rate, which is the number of calories your body burns at rest. That's partly why holding on to muscle matters for keeping weight off long term, not just for how you feel during treatment.3
The good news? Muscle lost during weight loss isn't necessarily gone for good. With consistent resistance training and enough protein, muscle can be rebuilt over time, even after a period of loss.3
How to protect your muscle on Mounjaro or Wegovy
Two things make the biggest difference. Neither means giving up on the medicine or overhauling your whole routine:
- Resistance training - working the major muscle groups on at least 2 days a week is the single most effective way to tell your body muscle is still needed, even in a calorie deficit.
- Enough protein - spreading protein across your meals helps support muscle repair, which is especially useful when a reduced appetite makes it easy to under-eat.
For a full breakdown of how much activity is recommended and which type of exercise matters most, see our guide on exercise on Mounjaro or Wegovy.
Start small. Two resistance sessions a week and some protein at every meal covers the basics.
When should you seek medical advice?
Speak to a GP or your prescriber if:
- You notice a rapid or unexplained decline in strength, such as struggling with stairs or carrying shopping that previously felt manageable
- You have unusually severe or persistent fatigue that does not improve after the first few weeks of treatment
- You have a pre-existing condition affecting bone density or muscle mass, such as osteoporosis or sarcopenia
Frequently asked questions
Does Ozempic cause muscle loss like Mounjaro or Wegovy?
It's the weight loss caused by semaglutide (Ozempic's active ingredient), not the drug acting on muscle
directly, that can reduce muscle mass, in the same way as Mounjaro and Wegovy. Note that Ozempic is licensed
in the UK for type 2 diabetes rather than weight loss, so Wegovy is the appropriate option for weight
management.
What percentage of weight loss on GLP-1 medication is muscle?
Clinical trial data suggests around a quarter of weight lost with tirzepatide is lean mass, similar to
weight loss from diet alone. This is considerably lower than the widely repeated claim of up to 40%, which
isn't supported by controlled trial evidence.
Is tirzepatide or semaglutide worse for muscle loss?
Trial data doesn't show a clear difference in the proportion of lean mass lost between the two. Some
real-world data suggests tirzepatide may be linked to slightly greater lean mass loss than semaglutide in
routine care, though this hasn't yet been confirmed in peer-reviewed research.
Does Ozempic cause facial muscle loss, sometimes called Ozempic face?
What's commonly called Ozempic face is caused by fat loss in the face rather than muscle loss
specifically. Rapid weight loss anywhere in the body can reduce facial fat, which may make some people look
more tired or aged, particularly if the weight loss happens quickly.
Can muscle loss on Mounjaro or Wegovy be reversed?
Yes. Muscle lost during weight loss is not permanent. With consistent resistance training and adequate
protein intake, muscle can be rebuilt over time, even if some was lost earlier in treatment.
Does resistance training prevent muscle loss on GLP-1 medication?
Resistance training doesn't eliminate muscle loss entirely, but it significantly reduces how much
muscle you lose compared to inactivity. Combined with sufficient protein intake, it's the most effective way
to protect muscle while losing weight on Mounjaro or Wegovy.
References
- 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. doi:10.1111/dom.16275
- Electronic Medicines Compendium. Mounjaro KwikPen 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg solution for injection in pre-filled pen, patient information leaflet. eMC; 2024. medicines.org.uk/emc
- Neeland IJ, et al. Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism. 2024. doi:10.1111/dom.15728
- Impact of Semaglutide on fat mass, lean mass and muscle function in patients with obesity: The SEMALEAN study. PMC. PMC12673431
- Electronic Medicines Compendium. Ozempic 1 mg solution for injection in pre-filled pen, summary of product characteristics. eMC. medicines.org.uk/emc
- Endocrine Society. Consuming more protein may protect patients taking anti-obesity drug from muscle loss. Presented at ENDO 2025. endocrine.org
- Greater lean-body-mass decline with tirzepatide than semaglutide in routine care, revealed by body-composition digital phenotyping. medRxiv; 2026 (preprint, not yet peer reviewed). medrxiv.org
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.