Why Do You Get Diarrhoea on Mounjaro or Wegovy?

Both Mounjaro and Wegovy work on GLP-1 receptors, which play a role in regulating digestion as well as appetite. One well-documented effect is that they slow gastric emptying — the rate at which food moves out of the stomach and through the gut. While this slowing is part of what helps people feel fuller for longer, it can also disrupt normal digestive rhythms, and for some people the result is loose stools or diarrhoea rather than constipation.

Mounjaro also acts on GIP receptors, making it a dual-action medicine. Both receptor pathways influence how the gut processes food and fluid, which is why gastrointestinal symptoms — including diarrhoea — are listed among the commonly reported effects for tirzepatide and semaglutide in their official product information.

When does it tend to happen?

Many people notice diarrhoea most in the day or two following an injection, particularly when starting the medicine or moving up to a higher dose. The gut is, in effect, adapting to a changed environment. Some find it eases within a few days; for others it lingers longer, especially at higher dose stages. If you are noticing a recurring pattern around your injection days, that timing is useful information to bring to your prescriber.

For a closer look at practical patterns and what others commonly experience, see our pages on diarrhoea during Mounjaro treatment and diarrhoea during Wegovy treatment. You may also find it helpful to read about managing loose stools alongside Mounjaro.

One of the most useful things you can do when a side effect like diarrhoea is recurring is to log it — not to self-diagnose, but to build a clear picture you can share with your prescriber. GLP Trackr's side-effect tracker lets you record symptoms day by day, so you can see whether episodes cluster around injection days, correlate with certain foods, or follow dose changes over time.

Because the app also logs your injections and dose history, you can spot patterns that would be easy to miss when you're simply living through them — for example, noticing that diarrhoea eased after the first two weeks at a new dose, or that it worsens when you eat certain things. That kind of context is genuinely useful when you sit down with your prescriber to review how the medicine is suiting you.

You can bring a clear summary of your logged symptoms directly to an appointment using the app's doctor report feature, so nothing important gets forgotten in the moment.

If diarrhoea is severe, persistent, or accompanied by signs of dehydration, contact your healthcare professional promptly — do not wait for your next scheduled appointment. GLP Trackr helps you record and reflect on your experience, but it is not a substitute for clinical advice, and your prescriber is best placed to advise you on managing side effects specific to your situation.

For a broader overview of what to expect during treatment, the guide to commonly reported Mounjaro side effects covers the full range of gastrointestinal and other effects that people on these medicines commonly experience.

Start logging your symptoms and injection days with GLP Trackr — available free on the App Store and Google Play, with a 7-day trial to get started.