Using GLP-1 medicines, there is a good chance of losing muscle mass. Over the long term, this could lead to problems such as osteoporosis. Because the bones, joints and muscles work closely together, the bones and joints are also put at risk and the risk of fractures rises.
Dr Mariella Borg Buontempo
GLP-1 medicines do cause some loss of muscle and other lean tissue, as other ways of losing weight do. The harm the speaker linked to it is not shown: large trials have found no rise in fractures overall, and the main trial on joints found less knee pain, not more.How we reached this verdict ↓
Analysis and evaluation
The claim is a chain: muscle loss leads to weaker bones and joints, and then to more fractures. The first link holds. The later ones are not shown by the trials so far.
Lean tissue falls with weight
Body scans in trials of GLP-1 medicines show lean tissue falling along with fat. A 2025 review in eClinicalMedicine puts lean tissue at 25% to 45% of the total weight lost, a share it describes as similar to weight loss through diet and exercise. The review notes the loss may matter for mobility and physical function, particularly in older adults.
Lean tissue is not all muscle: it also includes water and organs. And because fat falls faster, lean tissue makes up a larger share of the body afterwards. In the main semaglutide weight-loss trial, lean mass fell by 9.7%, but its share of body weight rose by 3 percentage points.
Bone density fell in one small trial; fractures did not rise in large ones
In a 52-week trial of 64 adults already at higher risk of fractures, those on semaglutide had lower bone density at the hip and spine than those on a placebo, and higher levels of a marker of bone breakdown. The authors suggest either less weight bearing down on the skeleton after weight loss or a direct effect of the drug.
Larger studies have not shown more fractures overall. A 2025 analysis of 25 trials in people with type 2 diabetes found no increase in fracture risk. In SELECT, which followed 17,604 adults with overweight or obesity and heart disease for over three years, fractures were about as common on semaglutide as on a placebo. There were more hip and pelvic fractures among women and people aged 75 and over on the drug, but the numbers were small. None of these studies measured osteoporosis diagnoses directly.
The main trial on joints found less pain
In STEP 9, 407 adults with obesity and knee osteoarthritis took semaglutide or a placebo for 68 weeks. Knee pain fell by 41.7 points on the drug against 27.5 on the placebo, on a 100-point scale, and physical function improved. We found no trial showing that GLP-1 medicines harm the joints.
Evaluation
What was said is partly right. Losing some muscle is a real effect of the weight loss these drugs produce. The bone risk is an open question, which the speaker's "could" reflects. But the claim that fractures rise and joints suffer is not shown by the trials so far, and on joints the main trial points the other way.
Editorial note
How we assessed this claim
We split the claim into the effect it describes and the harm it attaches to that effect, and checked each with randomised trials and reviews: muscle loss first, then bone density and fractures, then joints.
The established part holds: lean tissue falls as weight falls. The harm attached to it, more fractures and damaged joints, is not shown by the trials so far, which keeps the claim below True but lacks context. The speaker framed the long-term risk with "could", which keeps it above Misleading. That lands it at Not entirely accurate, the same reasoning used for the retatrutide claim in this episode.
Limitations
- The claim does not name a specific GLP-1 drug. Most of the long-term evidence is for semaglutide.
- SELECT was designed to measure heart outcomes, not fractures, and followed people for just over three years.
- The bone density trial was small and used a lower dose of semaglutide (1.0 mg) than the usual weight-loss dose (2.4 mg).
- The fracture analysis of 25 trials covered people with type 2 diabetes, not obesity alone.
- SELECT and STEP 9 were funded by Novo Nordisk, which makes semaglutide.
Sources
- Moiz A. et al., The expanding role of GLP-1 receptor agonists: a narrative review of current evidence and future directions, eClinicalMedicine (2025)
- Exploring beyond numeric weight loss: the metabolic effects of semaglutide, narrative review (2025)
- Once-weekly semaglutide versus placebo in adults with increased fracture risk: a randomised, double-blinded, two-centre, phase 2 trial, eClinicalMedicine (2024)
- Tan Y. et al., Effect of GLP-1 receptor agonists on bone mineral density, bone metabolism markers, and fracture risk in type 2 diabetes: a systematic review and meta-analysis, Acta Diabetologica (2025)
- Kushner R.F. et al., Safety profile of semaglutide versus placebo in the SELECT study: a randomized controlled trial, Obesity (2025)
- Bliddal H. et al., Once-weekly semaglutide in persons with obesity and knee osteoarthritis (STEP 9), New England Journal of Medicine (2024)