Weight loss jabs: The side effect nobody’s talking about. And how to avoid it

The hidden impact of weight loss drugs

The impact of weight loss drugs on skeletal muscle mass has been revealed in a new report by Les Mills and ukactive.

Led by Gillian L. Hatfield, PhD, Associate Professor at the University of the Fraser Valley, Canada, the report is one of the first reviews of global academic evidence exploring how weight loss medications work, their side effects, and strategies to prevent lean body mass loss.

The findings spell out the importance of preserving skeletal muscle and bone mass during weight loss, and the essential role of strength training. According to the authors, strength training should be a cornerstone of any weight loss prescription. They’re calling for collaboration between Government, pharmaceutical, nutrition, and fitness sectors to deliver sustainable support for millions using these medications.

“Millions of people are taking weight loss medications across the UK, but this evidence shows the risks of ignoring strength training and exercise during treatment,” says Dr Matthew Wade, Interim Director of Research at ukactive. “There are simple and effective strength exercises which are essential for sustainable results.”

What are GLP-1s – and how do they work?

Originally developed for the treatment of type 2 diabetes, GLP-1 (glucagon-like peptide 1) receptor agonists mimic a natural hormone to suppress appetite, promote weight loss, and reduce cardiovascular risk. They’re hailed as game-changers, but they’re not a cure. In the same way blood pressure medication eases hypertension but doesn't cure it, GLP-1 simply treats the issue by suppressing appetite. The need for lifestyle changes – like increased exercise and improved diet – is essential, because when you stop taking the medication, appetite returns, and weight-regain can be fast. A recent analysis shows those who stop using the medication have weight return four times faster compared with other weight loss plans.

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Those who stop using medication have weight return four times faster.

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While GLP-1s are effective for sparking weight loss (and the many health benefits that come with it), like all calorie-restricted approaches, GLP-1 drugs can cause significant lean body mass loss. Evidence shows 20–50% of weight lost is muscle and bone, which poses serious health risks.

Why muscle loss matters

When weight drops rapidly, the body often breaks down muscle for energy. For GLP-1 users, the magnitude of lean body mass loss can be on par with the loss experienced after bariatric surgery, cancer treatment, or 10 years of ageing.

Muscle plays a critical role in glucose metabolism, fat burning, and overall resilience. Losing it compromises health far beyond aesthetics.

Low muscle mass is linked to increased risk of cardiovascular disease and diabetes and is an independent risk factor for all-cause mortality. A 2025 meta-analysis highlighted a 30% higher risk of all-cause mortality in middle-aged and older individuals with low lean mass compared to those with normal lean mass. It is also directly linked to weakness and frailty, which is particularly concerning for older adults who already face an increased risk of falls, costing £2.3bn every year in theUK alone.  

How to protect muscle while losing weight

“Regular strength training is vital at all life stages, but  the research suggests this is especially true for people taking weight loss medications,” says Bryce Hastings, Head of Research at Les Mills.

Studies included in the report show that combining exercise – especially strength training – with a healthy diet helps maintain muscle, supports weight loss, and reduces regain after treatment ends. There is also evidence that strength training protects bone density too.

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Hastings recommends: “Gradually building up activity as side effects allow. Strength training two to three times a week, plus 150 minutes of moderate to vigorous activity weekly. Studies have shown this helps continue fat mass loss in the weight maintenance phase of treatment and lessen weight regain after treatment has stopped.”

Weight loss prescriptions in the future… More than just jabs

Hastings says that, while more research is needed to better understand the relationship between weight loss medications and strength training, this report is a key starting point. “It provides evidence and recommendations to start the journey and offer practical next steps to better support the growing number of people taking weight loss medication.”

Guidance from the National Institute for Health and Care Excellence (NICE), echoes this, suggesting that in the future, those who are prescribed medicines for weight management must receive wrap-around care focusing on diet, nutrition and increasing physical activity, including strength training.

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