Mounjaro and muscle loss: keep your strength
Losing weight on Mounjaro or Wegovy takes some muscle with it. What the research found, what UK guidance says, and how strength training helps you keep it.
By Anna & Charlotte · MNU-Certified Nutritionists · Level 3 Personal Trainers
Published 9 November 2026
TL;DR
Yes, some of the weight you lose on Mounjaro is muscle. In a study of people taking tirzepatide (Mounjaro), about a quarter of the weight lost was lean mass, the same share as in people losing weight without it. UK guidance says people on these medicines especially need activities that keep muscle strong, and that adults should do strength work on at least 2 days a week. Questions about your medicine go to your prescriber.
Some of the weight you lose on a weight loss medicine like Mounjaro or Wegovy is muscle. That's no reason to panic: in the main study, people losing weight without the medicine lost the same share of lean mass. What it does mean is lifting something heavy a couple of times a week, and this post explains why and how.
A quick note before we start. Anna and Charlotte are personal trainers and nutritionists, not doctors or pharmacists. Nothing here is advice about your medicine. Your dose, your side effects and whether a medicine is right for you are all questions for your prescriber.
Do you lose muscle on Mounjaro?
Yes, some. Mounjaro is the brand name for tirzepatide. Wegovy and Ozempic are brands of semaglutide; the NHS lists Wegovy for weight loss and Ozempic for type 2 diabetes. You can only get them on prescription, and the NHS says tirzepatide and Wegovy help you lose weight alongside exercise and changes to your diet (NHS: tirzepatide, NHS: semaglutide).
The clearest numbers we've found come from a body scan study inside SURMOUNT-1, a trial of 2,539 people taking tirzepatide or a placebo. 160 adults had DXA scans, which measure fat and lean mass separately, at the start and after 72 weeks (Look et al., 2025). Here's what changed.
| After 72 weeks | Tirzepatide | Placebo |
|---|---|---|
| Body weight | down 21.3% | down 5.3% |
| Fat mass | down 33.9% | down 8.2% |
| Lean mass | down 10.9% | down 2.6% |
| Share of the weight lost that was lean mass | about 25% | about 25% |
Two things stand out. First, most of the weight lost was fat, roughly three quarters of it. Second, the people on placebo, who were also losing weight, lost lean mass in the same proportion. The medicine didn't make people lose a bigger share of lean mass. It made them lose more weight overall, so more lean mass went in total. The study was funded by Eli Lilly, which makes Mounjaro, so read the numbers as one study's findings rather than the full picture.
What UK guidance says about exercise on these medicines
The UK Chief Medical Officers' physical activity guidelines, updated in July 2026, now talk about GLP-1 medicines (tirzepatide and semaglutide are both in this group). They say that when you take them "both fat and muscle decrease", and that people taking GLP-1 medicines "especially need to ensure they do activities to maintain muscle strength" (UK Chief Medical Officers' physical activity guidelines).
For adults, that same guidance says muscle strengthening activities "should be done on at least 2 days a week", alongside at least 150 minutes of moderate activity across the week. The NHS adult guidelines say the same, and count lifting weights, resistance bands and bodyweight exercises such as push-ups as strengthening. This guidance covers adults in the UK.
Why this matters more if you're over 40
aceSTRONG is designed for women in their 40s, 50s and beyond, and that's already the age when holding on to muscle takes more deliberate work. We go through why in muscle loss after 40. Add a large weight loss on top, and keeping the muscle you have becomes the thing worth protecting. Muscle is what lets you carry the shopping, get up off the floor and keep doing the things you enjoy.
aceSTRONG suits women on GLP-1 medicines who want to keep their strength, including those who know they need to build muscle but don't know where to start. It comes with guidance on how to progress your weights, so you can see your strength going up while the scales go down.
What a strength week can look like
You don't need a gym or a complicated plan. You need a few sessions a week that work your whole body and get gradually harder.
- Two or three sessions a week, spread out. Two is the UK minimum. Anna and Charlotte build aceSTRONG around three, which gives you more practice without taking over your week. More on that in how often a woman over 40 should lift weights.
- Work the big muscle groups. Legs, hips, back, abdomen, chest, shoulders and arms, which is the list the NHS gives.
- Make it a bit harder over time. A slightly heavier weight, an extra rep or a harder version of the move. If you're not sure where to start with weights, read how heavy your dumbbells should be.
- Write your weights down. It's the easiest way to see that you're getting stronger while the scales go the other way.
The NHS lists feeling sick and feeling tired among the common side effects of both tirzepatide and semaglutide. If those get in the way of training, that's a conversation for your prescriber, not something to push through.
Eating enough when your appetite is low
These medicines help you lose weight by reducing your appetite, and the NHS says you'll need to eat a healthy, balanced diet while you take them (NHS: tirzepatide, NHS: semaglutide). When you're eating less, it gets harder to eat enough protein, and Anna and Charlotte put protein first for keeping muscle.
Anna and Charlotte's own everyday rule is around 30g of protein at every meal, and we explain the thinking in why 30g of protein with every meal. That's their coaching rule for clients generally, not medical guidance for people on these medicines. If your appetite makes eating enough feel impossible, ask your prescriber or a dietitian what's right for you.
Where aceSTRONG fits
aceSTRONG is a 12-week strength programme designed for women in their 40s, 50s and beyond. It's 3 follow-along strength workouts a week that you can do at home or in the gym with minimal equipment, with guidance on how to progress your weights and the aceSTRONG community alongside you. It's a one-off payment of £99 for the 12 weeks. It's a programme you follow, not live coaching, and it sits alongside your prescriber's advice rather than replacing it.
If you're losing weight on Mounjaro or Wegovy and want to keep your strength while you do, take a look at aceSTRONG.
Frequently asked
Can you still build muscle on Ozempic?
The study we've looked at was of tirzepatide (Mounjaro), and it measured how much lean mass people lost, not how much muscle they built, so it doesn't answer this directly. What UK Chief Medical Officers' guidance does say is that people taking GLP-1 medicines, which include Ozempic, especially need activities that keep muscle strong, and that adults should do strength work on at least 2 days a week.
How do I stop losing muscle on Ozempic?
In the tirzepatide (Mounjaro) study we've looked at, some lean mass went with the weight loss whether or not people took the medicine. Strength work on at least 2 days a week, as the UK Chief Medical Officers advise for adults, is the part you control. Questions about your dose or diet go to your prescriber.
How to regain muscle after Mounjaro?
Anna and Charlotte's answer is regular strength training that gets a little harder over time; the UK minimum is 2 days a week. If you've stopped your medicine on medical advice, check with your GP or prescriber before starting a new exercise plan.
Is muscle weakness a side effect of Mounjaro?
Muscle weakness isn't on the NHS list of common tirzepatide side effects; feeling tired is. If you notice new weakness, speak to your prescriber or GP rather than training through it.
