Mounjaro muscle loss: 'No need to worry', says expert

Using a weight loss injection, such as Wegovy or Mounjaro, often leads to muscle loss. This is a fact.

However, there are many myths floating around the internet that make this complex subject simpler and scarier than it needs to be.

Myths like 'Muscle loss is inevitable' (wrong – with regular resistance training, you can retain or even increase muscle mass).¹ And 'Weight loss injections directly cause muscle loss' (likely wrong – experts believe it's the weight loss itself, not the injections, that cause muscle loss).²

An article by Dr Yoni Freedhoff, published earlier this month at Medscape, puts these myths under sharp scrutiny.

The gist of Freedhoff's argument is this. Weight loss injections do cause some muscle loss, but the effects can be mitigated and the benefits vastly outweigh the risks.

However, it's worth digging deeper into the details. Freedhoff makes some important points that might just change how you think about muscle and weight loss.

If you lose weight, you'll lose muscle – unless you do something about it

In his article, Freedhoff invites us to imagine carrying around a 50lb dumbbell for eight to 12 hours a day.

That's a lot of work for our muscles. And if you suddenly decided to stop carrying the dumbbell, those muscles would inevitably shrink. After all, they'd simply have less work to do.

This is more or less what happens when we lose weight. Carrying excess weight puts extra strain on our arms, legs, back and stabilising muscles. Our day-to-day lives become a constant workout – until finally, we lose weight and the workout stops.

The point is this: if you lose weight, you'll also lose muscle. And if you lose a lot of weight quite quickly – by using weight loss injections, for example – the effects are even more pronounced.

And the answer? Give those muscles something to do. By incorporating resistance training into your weight loss routine and eating plenty of protein, muscle loss can be successfully minimised.

Weight loss makes it easier to stay fit

We often think of muscular bodies as healthy bodies. But when it comes to weight loss, it's all relative. 

Freedhoff points to a recent study examining the effects of weight loss medications on muscle mass in humans and mice.

The study found that, among those taking weight loss medications:³

• Absolute muscle mass and strength decreased
• But relative muscle mass and strength increased
  
  

In simple terms, the participants lost some muscle while losing weight. However, muscle made up a larger percentage of their overall body composition afterwards. They were, relatively speaking, more muscular than when they started.

It's easy to see how this could make exercise easier. Freehoff cites anecdotal evidence from his own patients, reporting that those who had taken weight loss medications found exercising more comfortable and less painful.

'That shouldn't be surprising', he wrote. 'Do you think you'd be more or less active if you were or weren't carrying that 50lb dumbbell?'

There may be other motivations for promoting muscle loss myths

Are newspapers and influencers publishing scare stories about muscle loss because they're genuinely concerned for people's health? Or is there a more nefarious motivation at play?

Freedhoff believes it's likely the latter. And the reason? 'Motivated weight bias.'

In other words, society stigmatises weight. Despite the best efforts of healthcare professionals and campaigners, excess weight is often seen as a personal failing rather than a health concern. And the public, consequently, is motivated to find fault with medications that address this 'personal failing' – as if they offer an 'easy way out'.

'In the media's cases,' Freedhoff goes on to say, 'bias is also motivated to find fault because fault drives a public primed to fear obesity medications to more clicks and listens.'

So, while muscle loss on Mounjaro and Wegovy is real, it's wise to look beyond the headlines and ask, 'Why, exactly, are they saying this?'

Semapen is a UK-based provider of expert-led weight loss treatments, including Wegovy and Mounjaro online programmes. For more news and weight loss advice, follow our blog.

Sources

1. Lahav, Y. et al. (2026) "Resistance training as a key strategy for high-quality weight loss in men and women" Frontiers in Endocrinology, 16:1725500 https://doi.org/10.3389/fendo.2025.1725500

2. Prado, C.M. et al. (2024) "Muscle matters: the effects of medically induced weight loss on skeletal muscle" The Lancet Diabetes & Endocrinology, 12(11) https://doi.org/10.1016/S2213-8587(24)00272-9

3. Langer, H.T. et al. (2026) "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, 7(3) https://doi.org/10.1016/j.xcrm.2026.102665

This article was reviewed and approved by Lujain Alhassan, Bariatric Nutritionist, on 22 May 2026.