What are the side effects of GLP-1 drugs, and do they cost muscle?
Most side effects are in the stomach and fade; the muscle question is still being worked out.
Guess first
?
Which one is it?
The most common side effects of semaglutide are digestive: nausea, diarrhea, vomiting, constipation and abdominal pain. According to a drug summary, they are dose-dependent and mostly mild to moderate, and they typically peak during the first 8 to 12 weeks of dose escalation before waning over time. Whether the drugs cost muscle is less settled. A meta-analysis of 22 trials found that lean mass made up approximately 25% of the weight people lost on GLP-1 drugs, while a 2024 review of six semaglutide studies found that the proportion of the body that is lean still increased. These are prescription medicines, and this lesson describes what sources report, not what anyone should do.
Why the stomach? The drug delays gastric emptying, which can lead to indigestion and belching and, rarely, to or bowel obstruction. Other effects are rarer but are tracked by regulators. Semaglutide is associated with an increased risk of gallstones, and the US label carries a for thyroid C-cell tumors that is based on rodent studies, since rodents have a high density of GLP-1 receptors in those cells. In 2026 the UK regulator MHRA updated its guidance after more reports of acute , some fatal, with semaglutide or tirzepatide, to warn of a small risk of severe acute pancreatitis.
Most common
- Nausea
- Diarrhea and vomiting
- Constipation
- Abdominal pain
- Mostly mild to moderate, then waning
Rarer, tracked by regulators
- Gallstones
- Severe acute (UK MHRA, 2026)
- NAION eye condition, very rare (EMA, June 2025)
- , rarely
- Thyroid (rodent studies)
Safety signals get checked. In July 2023 Iceland's medicines agency reported cases of suicidal thoughts among users, and a safety assessment followed. A preliminary FDA review in January 2024 found no evidence that the medicine causes suicidal thoughts or actions, and in January 2026 the FDA requested removal of the warning on these medicines.
On muscle, note what "lean" means. Lean body mass is total body weight minus body fat, so it counts everything that is not fat, not just muscle. That is one reason the muscle question is hard to answer from the numbers alone.
Quiz me
0/3
Recap
Digestive effects are the most common and often fade; the muscle effect varies between studies.
💡 A trick to remember it · Like a rough sea at the start of a voyage, the stomach churns early and then settles; how much muscle goes overboard depends on which study you read.
Surprising fact · Lean body mass counts everything that is not fat, so a drop in it is not the same as a drop in muscle.
Connects to
- 📈 How did diabetes drugs become weight-loss drugs, and what did the trials find?
- 🔁 What happens to weight when people stop taking GLP-1 drugs?
- 💉 How do Ozempic, Wegovy, Mounjaro and Zepbound work?
- 🏃 Why are some muscle fibres fast and others slow?
- 🧪 Why do nine in ten drugs that reach human trials never get approved?
Sources (6)
No source, no claim. Every fact in this lesson (17 claims) cites at least one of these.
- [1]Semaglutide · Wikipedia
- [2]Tirzepatide · Wikipedia
- [3]Lean body mass · Wikipedia
- [4]Anti-obesity medication · Wikipedia
- [5]A systematic review of the effect of semaglutide on lean mass: insights from clinical trials (Expert Opinion on Pharmacotherapy, 2024), abstract · Expert Opinion on Pharmacotherapy (via PubMed)
- [6]Effect of glucagon-like peptide-1 receptor agonists and co-agonists on body composition: systematic review and network meta-analysis (Metabolism, 2025), abstract · Metabolism (via PubMed)