While fat loss is the key aim when using weight loss medications, there is growing concern that GLP-1 drugs might be causing people to lose lean muscle mass and worsen their body composition. Here’s everything you need to know about why it happens – and how to preserve your muscle mass while using weight loss medications.
The number of people using weight loss drugs has soared in recent years, with millions of people around the world opting to use GLP-1 medications to lose weight quickly. And while fast fat loss is the goal, you might also be sacrificing your lean muscle mass along the way.
Research shows that popular weight loss medications, including jabs such as Wegovy, Ozempic, and Mounjaro, do not discriminate between fat and muscle when helping people to shed weight. In fact, a study reported in The Lancet Diabetes & Endocrinology shows 25 to 39 per cent of the weight lost while using medications consists of muscle mass.
What’s more, stopping weight loss medications has also been shown to be followed by rapid weight regain. And while research into how body composition changes after stopping weight-loss drugs is ongoing, experts believe the weight commonly regained after coming off the drugs will primarily consist of fat – unless healthier diet, fitness and lifestyle changes are made, as part of a longer-term weight management strategy.
Do weight loss jabs cause muscle loss?
In short: yes. While fat loss is the key aim when taking weight loss medications, research shows that muscle loss is a common side effect. And the cause often comes down to a lack of awareness around healthy nutrition.
‘The medication quiets hunger, but it doesn’t teach people how to eat in a way that protects muscle, supports hormones, or sets them up for long-term success,’ explains Melanie Murphy Richter, dietitian and expert in longevity and fasting at L-Nutra. ‘There’s little education around behaviour change, meal structure, or metabolic preservation. That educational gap is where many problems begin.’
‘One of the biggest issues I see clinically is unintentional under-eating,’ she continues. ‘People aren’t just eating less – they’re often not consuming enough protein, total energy, or key nutrients. Early side effects like nausea, reflux, bloating, constipation, fatigue, or dizziness are common, but the more concerning effects tend to show up quietly over time. This includes nutrient deficiencies and a loss of lean muscle mass.’
How your body responds to under-eating and muscle loss
A common misconception is that under-eating will result in fast and effective fat loss. But according to Melanie, failing to fuel yourself correctly can majorly disrupt almost every system in your body.
‘When the body is consistently under-fed, it doesn’t simply “burn fat”,’ she explains. ‘It adapts in ways that affect nearly every system. It’s common to experience hormonal disruption, menstrual cycle changes, reduced libido, hair thinning, skin changes, brittle nails, brain fog, mood changes, bone density loss over time, and a slowed metabolic rate driven by muscle loss.’
Losing muscle is common side effect of using weight loss medications – and according to Melanie, the consequences go far beyond a loss of physical strength. ‘Muscle is one of the most important organs for longevity,’ she explains. ‘It plays a critical role in blood sugar regulation, insulin sensitivity, hormone balance, and long-term metabolic health. Most people want fat loss, particularly visceral fat around the organs, but GLP-1 medications don’t distinguish between fat and muscle.’
Why weight gain is so common after stopping medication
It’s normal for weight to fluctuate, and weight regain is common for anyone who has lost a significant amount of weight – drugs or no drugs. However, one study demonstrates that weight regain occurs significantly faster after stopping GLP-1 drugs than it does after stopping a behavioural weight management programme.
‘Rebound weight gain is common, and it’s not a personal failure – it’s physiology,’ says Melanie. ‘When GLP-1 medications are stopped, appetite returns – often quickly – hunger hormones rebound, metabolic rate may be lower due to muscle loss and any unaddressed eating patterns or stress behaviours resurface.’
Despite weight loss medications causing a combination of muscle and fat to be lost, it’s common for the weight regained after stopping the drugs to primarily consist of fat. ‘If muscle mass has declined, the body burns fewer calories at rest, making weight regain more likely and often faster, frequently with a higher proportion of fat,’ Melanie explains. ‘This is why muscle preservation during weight loss is so critical. Long-term success depends on rebuilding nourishment, muscle, and behaviour, not simply removing the drug.’
How to preserve your muscle while using weight loss medications
Unless you take steps to protect your muscle mass while on GLP-1 drugs, you might end up worsening your overall body composition by encouraging a higher fat to muscle ratio. According to Melanie, building a long-term strategy is the key to avoiding this.
‘Protecting health while on injections means being proactive,’ she explains. ‘Focus on prioritizing protein even when appetite is low, avoiding long stretches of eating very little or nothing, supporting micronutrient intake, incorporating resistance or strength-based movement when appropriate, and monitoring strength, energy and metabolic markers – not just the scale.
‘For many people,’ she continues, ‘GLP-1 medications are not short-term tools. Long-term or indefinite use is often required to maintain weight loss. However, costs can be significant and ongoing, insurance coverage and access vary widely and long-term safety data is still evolving.’
‘That’s why many clinicians are now thinking beyond “how do we lose weight quickly?” and asking “how do we support metabolic health over time?”. Nutritional frameworks like fasting-mimicking diets, strength preservation, and behaviour-based education are increasingly part of that conversation. Not as replacements for medication, but as ways to reduce dependency and improve long-term outcomes.
‘At the end of the day, weight loss is a moment – metabolic health is a trajectory,’ Melanie concludes. ‘Protecting muscle, nourishing the body, and learning how to eat well – during and after medication – is what determines whether results last.’
