The scale is moving. So why do you feel weaker?

If you're on a GLP-1 medication like semaglutide (Ozempic or Wegovy) or tirzepatide (Mounjaro or Zepbound) and you've noticed you're losing weight but feeling weaker, more tired, or looking 'softer' despite the numbers going down — you're not imagining it. There's a clinical reason for it, and it's one of the most under-discussed side effects of these medications.

What the research actually shows

A systematic review published in PubMed examined 1,541 patients across six semaglutide studies and found that lean mass reductions ranging from near zero to 40% of total weight lost were observed depending on the individual. A 2025 report from the American Diabetes Association confirmed the same figure: as much as 40% of weight lost on GLP-1s can come from lean muscle rather than fat.

That number matters more than it sounds. Muscle isn't just about how you look or how strong you are. It's the primary driver of your resting metabolic rate — the number of calories your body burns just to stay alive. When you lose muscle, your metabolism slows. And a slower metabolism creates a dangerous setup for what happens next.

The weight regain trap

The STEP 1 trial extension — one of the largest studies on semaglutide outcomes — found that participants regained two-thirds of their prior weight loss within one year of stopping the medication. A 2025 meta-analysis in eClinicalMedicine found that people stopping semaglutide or tirzepatide regained an average of nearly 10 kg within the first year after stopping.

Here's what most people aren't told: a significant part of that regain risk comes from the muscle loss that happened during treatment. When your metabolic engine is smaller, your body simply can't sustain the weight loss without the drug doing the work. The medication was compensating for the muscle you lost.

Why this happens on GLP-1s specifically

GLP-1 medications suppress appetite so effectively that most people dramatically reduce their caloric intake — often below what the body needs to maintain muscle tissue. When protein intake drops and resistance training isn't in place, the body turns to muscle as a fuel source. It's not the medication itself destroying muscle. It's the nutritional gap the medication creates.