Muscle loss is one of the most common concerns people raise before starting Mounjaro or Wegovy. It's a valid 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 medications acting on muscle directly, that can affect how much you keep. Despite what's commonly cited online, the evidence on how much is more reassuring than you may have thought, although 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 is most at risk of losing more than average, how Wegovy and Ozempic compare, and what you can do to protect muscle while losing weight.
Does Mounjaro cause muscle loss?
Not directly. Mounjaro doesn't cause muscle loss through any direct effect on muscle tissue. What it causes is significant weight loss, and it's the weight loss itself, not the medication, that can reduce muscle mass, in the same way any substantial weight loss can, regardless of what caused it.
In the SURMOUNT-1 trial, the pivotal study behind Mounjaro's approval, researchers measured body composition directly using DXA scanning rather than just tracking weight on a scale. Of the total body weight lost with tirzepatide, around three quarters was fat mass and about a quarter was lean mass, a split that was almost identical to the placebo group, who lost a similar amount of weight through diet and lifestyle changes alone rather than medication.1
That similarity is telling. It suggests the muscle loss tracks with the amount of weight lost, not with Mounjaro acting on muscle independently of that weight loss.
Why does Mounjaro, or any GLP-1 medication, cause muscle loss?
Mounjaro and Wegovy don't target muscle directly. They reduce appetite and slow gastric emptying, which leads to eating less and a calorie deficit.2 When your body is in a calorie deficit for a sustained period, it draws energy from stored fat, but it also breaks down some muscle tissue for fuel. This happens with any method of weight loss, whether that's diet alone, exercise alone, or medication.
The rate at which this happens can be influenced by how quickly you lose weight, how much protein you eat, and how physically active you are. Researchers who reviewed the evidence on lean mass changes during GLP-1 treatment have pointed out that there is no single fixed rule for how much muscle any one person will lose. The proportion varies with total energy intake, diet composition, sex, starting body composition, and activity levels, not the medication dose alone.3 That's why two people using the same dose of Mounjaro can end up with quite different outcomes.
Does Wegovy cause muscle loss?
The same is true of Wegovy, and for the same reason: it's the weight loss it causes, not the medication acting on muscle directly, that accounts for any reduction in lean mass. Wegovy contains semaglutide, and in a dedicated body composition study of people using semaglutide 2.4 mg, lean mass fell in the first seven months of treatment before stabilising, while handgrip strength (a measure of muscle function, not just muscle size) actually improved by around 4.5 kg by the twelve month mark.4 The same study found the proportion of people classed as having sarcopenic obesity, a combination of high fat mass and low muscle function, fell from 49% at the start of treatment to 33% after a year.
In other words, some early lean mass loss alongside weight loss on semaglutide does not necessarily mean your muscles are getting weaker overall, particularly once your body adjusts and strength training or protein intake come into the picture.
Does Ozempic cause muscle loss?
This is a common query, 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 formulation that's licensed specifically for weight management.
Because they share the same active ingredient, the body composition evidence for semaglutide broadly applies to both, though the doses used for diabetes management are typically lower than the doses used for weight loss. As with Mounjaro and Wegovy, any muscle loss associated with Ozempic would come from the weight loss it causes, not from 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 do not apply evenly to everyone. A few groups tend to lose a larger share of muscle than the figures above suggest:
- Older adults - muscle mass naturally declines with age, so there is less to spare and less capacity to rebuild it quickly.
- Women - because women typically start with less skeletal muscle than men, the same absolute amount of muscle loss represents a larger proportional loss.
- 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 - a faster rate of weight loss is consistently linked to a higher proportion of that weight coming from lean mass, regardless of what's causing the weight loss.
A study presented at the Endocrine Society's 2025 annual meeting compared people using semaglutide against people following a diet and lifestyle programme alone. The proportion of weight lost as lean mass was similar between the two groups overall, but within the semaglutide group, being older, being female, or eating less protein were each linked to greater muscle loss. Losing more muscle was, in turn, linked to smaller improvements in blood sugar control.6 None of these risk factors are fixed. 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 figures online claiming that up to 40% of the weight lost on GLP-1 medication is muscle. The controlled trial data does not support a figure that high. As covered above, the SURMOUNT-1 body composition sub-study found lean mass made up around a quarter of total weight lost with tirzepatide, in line with placebo.1
Real-world data tells a slightly more complicated story than the trial data alone. An early analysis of routine health records from over 670,000 people using semaglutide or tirzepatide, which has not yet been through full peer review, found that a meaningful minority experienced notably greater lean mass loss than trial participants, particularly people with existing joint pain or reduced exercise tolerance before starting treatment.7 The strongest predictor of greater muscle loss in that analysis was not the medication or the dose, it was how much exercise capacity someone had to begin with.
That detail matters. Clinical trials include structured advice on protein intake and activity as standard, which real-world treatment does not always include to the same degree. It's one of the clearest pieces of evidence that staying active during treatment, rather than the medication itself, is what determines how much muscle you keep.
Does it actually matter if you lose some muscle?
Muscle is not just about how your body looks. It's metabolically active tissue, meaning it burns energy even at rest, and it plays a role in blood sugar regulation, bone health, and everyday tasks like climbing stairs or carrying shopping. Losing a meaningful amount of it can lower your resting metabolic rate - the number of calories your body burns at rest. That's partly why muscle preservation matters for keeping weight off in the long term and not just for how you feel during treatment, as it helps keep your resting metabolic rate higher.3
Muscle lost during weight loss is not necessarily gone for good though. With consistent resistance training and adequate 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, and neither requires giving up on the medication or overhauling your entire routine:
- Resistance training - working the major muscle groups on at least 2 days a week is the single most effective way to signal to your body that muscle is still needed, even in a calorie deficit.
- Adequate protein intake - spreading protein across your meals helps support muscle repair, particularly useful when appetite suppression makes it easy to under-eat overall.
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.
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.