Why does a GLP-1 medicine cause constipation?

Constipation appears on the label for both semaglutide and tirzepatide as a commonly reported effect. These medicines slow how quickly the stomach empties, and reduced appetite often means eating less food and drinking less fluid, both of which change bowel habit.

General guidance on constipation from the NIDDK and the NHS covers fibre, fluid and physical activity, and describes when a change in bowel habit is worth medical attention rather than self-management. That guidance is written for constipation broadly, not specifically for people on these medicines.

That distinction matters. Whether a particular laxative or fibre supplement is appropriate alongside a medicine that already slows gastric emptying is a question for your prescriber or pharmacist. Bring the pattern, not just the symptom: when it started, whether it followed a dose change, and what you have already tried.