GLP-1 Medications and Muscle Loss: What the Evidence Shows

Worries about GLP-1 medications and muscle loss are common, and the short answer is that some muscle and other lean tissue is lost along with fat, as happens with most substantial weight loss. In the studies behind Mounjaro, Wegovy and Foundayo, most of the weight lost was fat. Less settled is how much the lean loss matters for long-term strength, especially in older people.

This guide is information only, not medical advice, and does not recommend any product or supplier. All four medicines are prescription-only in the UK, and a qualified prescriber decides whether any of them suits you. New to the class? Start with what GLP-1 medications are and how they work.

GLP-1 medications and muscle loss: why it happens

Carrying extra weight tends to build some extra lean tissue, partly because your muscles work harder to move the heavier load. When weight comes down, some of that lean tissue goes too. Dieting studies commonly find that roughly 75% of the weight lost is fat and 25% is lean mass.

GLP-1 medicines work mainly by reducing appetite, so people eat less. The UK prescribing information for all four says each should be used alongside a reduced-calorie diet and increased physical activity. That second half is where muscle comes in.

Lean mass is not the same as muscle

Most of the studies used a DXA (DEXA) scan, a low-dose X-ray that separates the body into fat, bone and lean mass. Lean mass is everything that is not fat or bone mineral: muscle, but also organs, tendons, ligaments and water. So a fall in lean mass is not all muscle, and none of these scans measured strength.

What the body-composition studies found

For three of the four medicines, a smaller sub-study scanned some trial participants before and after treatment. These were separate trials with different people and methods, so the results should not be ranked.

The published SURMOUNT-1 analysis found the roughly 75:25 split held for men and women and across age groups, including people aged 65 and over, although several subgroups were small. Drug-by-drug detail is on our Mounjaro, Wegovy, Wegovy pill and Foundayo pages.

Does the muscle loss matter?

Researchers read the numbers differently, and the long-term picture is unclear.

• The reassuring reading: on tirzepatide, the share of weight lost as lean mass matched placebo and dieting studies, so the medicine did not appear to remove lean tissue out of proportion to the weight lost. In the Mounjaro, Wegovy and Foundayo trials, people also reported bigger improvements in physical functioning than those on placebo.
• The cautious reading: a 2025 review in Obesity Reviews estimated that trial participants lost 10% or more of their muscle mass over 68 to 72 weeks, warned this can reduce physical function, metabolic health and quality of life, and recommended protein and resistance-training advice for everyone treated.

All this evidence has limits: the sub-studies were small (55 to 171 people), all four trials were sponsored by the manufacturers, the review was funded by a nutrition company, physical functioning was self-reported, and follow-up lasted 68 to 72 weeks.

Who may need to take more care?

Muscle naturally declines with age, and marked loss is called sarcopenia. The SURMOUNT-1 analysis notes that losing lean mass may be undesirable for older adults who have, or are at risk of, low muscle mass alongside obesity. The Wegovy injection and tablet leaflets list hip fractures as a very rare side effect; in one large trial, hip and pelvis fractures were more common on Wegovy than placebo in women and people aged 75 and over. If you are older, frail or have had falls, raise this with your prescriber before and during treatment. Suitability rules are covered in who is eligible for GLP-1 treatment in the UK.

How to protect muscle while taking a GLP-1 medicine

• Strength activities on at least 2 days a week. The NHS physical activity guidelines advise adults to work all the major muscle groups (legs, hips, back, abdomen, chest, shoulders and arms) at least twice a week, on top of 150 minutes of moderate activity a week (or 75 minutes of vigorous activity). Examples include lifting weights, resistance bands, push-ups and heavy gardening. Adults aged 65 and over are advised to do activities that improve strength, balance and flexibility on at least 2 days a week.
• Protein foods at meals. With a smaller appetite, what you do eat matters more. The NHS Eatwell Guide suggests including beans, pulses, fish, eggs, meat and other protein foods, plus some dairy or alternatives. Protein needs vary, so ask your prescriber, GP or a registered dietitian what is right for you.
• Build up sensibly. The NHS suggests speaking to your GP first if you have not exercised for some time or have medical conditions or concerns.
• Use the support on offer. NICE says diet, activity and behaviour support should be arranged when a weight-management medicine is prescribed, so ask what comes with yours.

If stomach side effects are making it hard to eat regular meals, our guide to managing GLP-1 side effects may help.

When should I speak to my prescriber?

Muscle loss is not listed as a side effect in the UK patient leaflets, but tiredness is, and the Wegovy leaflets list feeling weak or tired as very common. Weakness can also signal low blood sugar if you take insulin or a sulfonylurea. Tell your prescriber if you feel noticeably weaker, find everyday tasks harder, or are struggling to eat enough. Do not stop or change your dose yourself. You can report any suspected side effect to the MHRA Yellow Card scheme. For wider differences between the medicines, see our comparison page, GLP-1 medications compared or the FAQ page.

Frequently asked questions