Does Mounjaro cause muscle loss?

Mounjaro primarily helps you lose body fat. However both can be lost, although most of the weight lost in the body-composition should be fat.

In adults with overweight or obesity, roughly 75% of the lost weight was fat, and around 25% was lean tissue.¹

A control group taking a placebo lost a similar amount of lean tissue, but people taking Mounjaro lost more weight overall. So people losing weight with Mounjaro lose more fat as a percentage, than muscle.¹

Is lean mass the same as muscle?

No. Lean mass includes muscle, organs and water. The scans which are used in this research measured the wider group of tissues, not just muscle alone.¹

This does not mean a quarter of your muscles have disappeared, only that a quarter of the total weight you lose is from lean tissue.

Mounjaro reduces your appetite and helps you feel full for longer. So when you are eating less, getting enough, and the right type of nutrition needs more attention.³

Your diet still needs protein and a balance of other foods. If smaller meals have reduced your protein intake, this can contribute towards muscle loss. You should bring those foods back into your routine, and pay specific attention to your protein intake.⁴

Feeling sick, being sick or having diarrhoea can make eating and drinking harder too. If the side effects are stopping you from eating properly, or are not settling, speak to your pharmacist or GP about what you can do. You can also read out Mounjaro side effects guide.³

Research has not shown that Mounjaro directly breaks down muscle. But it remains important to pay attention to what you eat and your activity levels.

Include protein in your meals and make strengthening exercise part of your day and week. These support muscle health during weight loss. ⁴

You do not need a complicated meal plan to start. Just try including a protein food in each meal, such as eggs, fish, chicken beans, lentils or tofu.⁵ High protein low fat sources include tuna, chicken breast, shrimp and turkey breast.

If you fill up quickly, try to eat the protein part of your meal first.⁵

Keep vegetables and carb foods in your diet too, including wholegrain options that you enjoy.⁴

For more everyday food and exercise ideas, read our guide to preventing muscle loss while losing weight.

How much protein do you need?

There's no single personal target for everyone. For the general population, 80–100g of protein a day is a suggested guide during weight-loss treatment. Your own needs might differ.⁵

If you have chronic kidney disease, you should ask your dietitian for a personalised protein target before increasing your intake.⁵

If you are struggling to reach an agreed target through food, protein shakes can be an option. However you do not need to buy one simply because you take Mounjaro.⁵ They can be helpful however if maintaining a high protein diet is difficult, particularly you are vegan.

If you are looking for ideas for balanced meals, our guide to what to eat on Mounjaro gives you more options.

Make strengthening exercise part of your week

For adults aged 19–64, aim for strengthening activity on at least two days per week. You should work your major muscle groups, including your legs, back, chest and arms.⁶

Options include weights, resistance bands and exercises using your own body weight. Choose an activity and a level of effort that suits your fitness.⁶

Walking and other aerobic activity can remain part of your routine. Include strengthening work, and ask for advice on suitable exercises if you have health or mobility limitations.

No. Tiredness can be a side effect of Mounjaro, and weakness can have other explanations. A new symptom needs to be assessed rather than just assuming muscle loss.³

If weakness persists or familiar tasks are becoming harder, such as climbing stairs or getting out of a chair, let your GP know. You should discuss how much you are eating, your weight change and any other symptoms.

Read our article on tiredness on Mounjaro covers that symptom in more detail.

When to get urgent help

Being sick or having diarrhoea can leave you dehydrated during Mounjaro treatment.⁷

• Get help from NHS 111 or ask for an urgent GP appointment if you are peeing less than usual, or dizziness on standing does not go away. These can be signs of serious dehydration.⁸
• Low blood sugar can cause weakness, sweating or a racing heartbeat. This risk is greater with insulin or diabetes medicines called sulphonylureas. Contact your GP promptly if these symptoms occur.⁷
• Get medical help immediately if you have severe stomach or back pain that doesn't go away. This can be a sign of an inflamed pancreas.⁷

• Make protein a deliberate part of your meals, even when portions are smaller.
• Plan regular strengthening activity that suits your fitness.

• Use protein shakes if you struggle to eat enough protein