On Mounjaro or Ozempic? The Muscle-Loss Trap Nobody Warned You About

Roughly 1.6 million adults in the UK are now on a weight-loss jab. If you're one of them, this is the most important thing nobody handed you with the pen.

Let me be straight with you, because it matters. I'm not a fan of these jabs. I don't like how fast they're handed out, I don't like how little people are told about the risks, and I don't trust a drug we've only been using this way for a few years to have no surprises waiting down the line. That's my honest position and I'm not going to dress it up.

But I'm also not here to lecture you or judge you. If you've decided a jab is right for you, that's a decision for you and your doctor, not me. What I can do is help you come out the other side strong instead of wrecked. So this article is two things: the side effects you genuinely should know about, and then the one problem I can actually fix for you as a trainer, which is the muscle you're losing without realising it.

The Side Effects You Don't Get Told About

Here's what bothers me most. Most people get these drugs after a quick online form or a ten-minute appointment, and walk away thinking the only downside is a bit of nausea. The real list is longer than that, and some of it is serious. I'm not saying this to frighten you. I'm saying it because you deserve to make an informed choice, and a lot of people simply aren't given one.

• Sudden vision loss. Regulators now take this seriously enough to put it on the label. In February 2026 the UK's medicines regulator, the MHRA, updated semaglutide's safety information over a rare eye condition called NAION, which cuts blood flow to the optic nerve and can cause sudden, sometimes permanent, loss of vision in an eye. Europe's regulator classes it as a "very rare" side effect, around 1 in 10,000, and it's now a documented risk on the label. Rare, yes. But real, and permanent when it happens.
• Bone loss. Fast weight loss strips bone as well as muscle. A 2024 trial found a year of semaglutide reduced hip bone density by roughly 2.6% and spine density by roughly 2.1%, and the drug's own label flags a higher fracture risk in older adults and women. That's a genuine worry, and it gets worse through the menopause.
• Stomach paralysis and gut damage. In America there are now thousands of lawsuits — more than 3,800 by mid-2026 — from people alleging these drugs caused gastroparesis, where the stomach stops emptying properly, along with other severe gut problems. A separate batch of cases is over the vision loss above.
• The long-term unknown. We don't have decades of data on using these drugs for weight loss at this scale. Anyone who tells you the long-term picture is completely settled is guessing.

So that's the backdrop, and it's why I'd always rather help someone get strong the hard, honest way first. But I'm a realist too. Plenty of people are going to take these drugs whatever I think — and if that's you, I'd far rather you did it as safely and smartly as possible. Which brings us to the one part of this I can genuinely fix: the muscle you lose without realising it.

Where Your Weight Actually Goes

When you lose weight, it can come off as fat or as lean tissue — mostly muscle. You want it to come off as fat. Obviously. The whole reason you're doing this is to be leaner and healthier, not just lighter.

Here's the part that gets glossed over. In the big trials, a meaningful slice of the weight people lost on these drugs was lean mass, not fat. On tirzepatide (that's Mounjaro), around a quarter of the total weight lost was lean tissue. On semaglutide (Ozempic and Wegovy), it ran even higher in some studies — closer to 40%. Read that again. Up to four in every ten pounds, gone as muscle.

Now, why does losing a bit of muscle matter so much? You didn't sign up to be a bodybuilder. Fair. But muscle isn't about looking big. Muscle is what holds your shape, keeps you strong and independent as you age, protects your bones, and — this is the killer — it's the tissue that keeps your metabolism ticking over. Muscle is metabolically expensive. It burns calories just existing. Lose a load of it and your body now needs fewer calories to run, which makes staying lean harder for the rest of your life.

So the person who loses two stone the careless way ends up lighter but "skinny fat": soft, still a high body-fat percentage, weaker than before, and sitting on a slower metabolism. The person who loses the same two stone while protecting their muscle ends up genuinely leaner, stronger, and with an engine that still works. Same number on the scale. Completely different body, and completely different odds of keeping it.

Two Things Are Making It Worse

The muscle loss isn't just an unlucky side effect of the drug. Two very ordinary things stack on top of it and make it far worse than it needs to be.

1. The appetite drop steals your protein

The jab works by killing your appetite. Brilliant for eating less. The problem is that when people eat less, protein is usually the first thing to go. It's the macro that takes effort to hit even when you're hungry, and when a plate of chicken feels like a mountain, most people just push it away and nibble a bit of toast instead. Protein is the exact nutrient your body needs to defend its muscle. Cut it right when the drug is already encouraging muscle loss, and you've kicked the door wide open.

2. You start moving less without realising

You'd think dropping weight would make you want to move more. A 2026 study that tracked real GLP-1 users with fitness trackers found the opposite: after starting the jab, people took around 560 fewer steps a day and did a few minutes less exercise. Lower energy, less appetite, less fuel — you just do less without noticing. And your muscles run on a simple rule: use them or lose them. Move less, and the body reads it as "we don't need all this muscle," and lets it waste away faster.

Put it together and you get a nasty little chain reaction.

And Then You Stop Taking It

Here's the bit that turns a cosmetic issue into a real one. Most people don't stay on these jabs forever. The cost alone — private Mounjaro runs anywhere from around £200 to £340 a month depending on dose — means a lot of people come off at some point. And when they do, appetite comes roaring back.

In the trials, people who stopped the medication regained a big chunk of the weight — in one well-known study, an average of around 14% of body weight came back after switching off the drug. Now here's the cruel twist. The weight you lost was part muscle. The weight you regain is almost entirely fat. So each time round the cycle, your body composition gets a little worse: less muscle underneath, more fat on top, at the same scale weight. That's how someone can lose and regain the "same" three stone twice and end up softer and weaker than when they started.

The Fix Is Almost Boringly Simple

Good news: you don't need anything clever. You don't need a second supplement stack or a special "GLP-1 protocol" someone's trying to sell you. You need to do the two things the drug quietly talks you out of doing. Eat enough protein, and lift weights. That's it. That's the insurance.

Do those and you flip the whole thing on its head. Instead of the jab stripping your muscle, you're keeping it — which means the weight you lose is overwhelmingly fat, your metabolism stays high, and if you ever come off the medication you're coming off with a strong, lean body that holds its result instead of springing back. You've turned a short-term fix into a permanent change.

Where I Come Into It

This is genuinely one of the most common conversations I'm having with new clients right now. Someone's a stone or two down on Mounjaro, delighted with the scale, but they feel weak, a bit deflated, and deep down they're worried about what happens when they stop. They're right to be. And the fix is exactly what I do all day: build a simple, sustainable strength and nutrition plan around a real person's life.

You don't have to choose between the jab and training. The smartest thing you can do is run them together — let the medication handle your appetite while we make sure the weight you lose is the right kind, and that what's left underneath is strong. That's how you come out the other side genuinely transformed, not just smaller.

Quick FAQ

Key Takeaways

• I'm honest about it: I'm not a fan of the jabs, but if you're going to take them, I'll help you do it properly.
• The side effects are under-sold: a rare but serious vision-loss risk (now on the label), bone loss, thousands of US lawsuits, and an unknown long-term picture. Go in informed.
• A big share of the weight lost on Mounjaro and Ozempic can be muscle: around 25% on tirzepatide, up to 40% on semaglutide.
• Appetite loss cuts your protein, and people take around 560 fewer steps a day — both speed up muscle loss.
• Lose muscle and you slow your metabolism, which sets up a worse rebound when you stop the drug (around 14% regain, mostly fat).
• The fix is simple: eat 1.6–2.2 g/kg of protein and lift weights 2–3 times a week. That's your insurance policy.
• Run the jab and training together and you come off with a lean, strong body that holds its result — not a smaller, softer one.

Disclaimer: This article is general education from a personal trainer, not medical advice. GLP-1 medications like Mounjaro, Ozempic and Wegovy are prescription drugs. Never start, stop or change your dose without talking to the doctor or prescriber who looks after your treatment. If you have any medical condition, get personalised advice before changing how you eat or train.