Mounjaro Causing Diarrhoea: Why It Happens and What to Do

Mounjaro works by activating both GLP-1 and GIP receptors, pathways involved in appetite regulation and digestion. One well-documented effect of GLP-1 receptor agonists is slowing gastric emptying — the rate at which food moves from the stomach into the intestine. During the adjustment period, this change in how the gut handles food can tip in the other direction for some people, causing looser stools or diarrhoea rather than the constipation others experience.

When does it typically happen?

Diarrhoea is most commonly reported in the early weeks of treatment or shortly after a dose increase. For many people it is short-lived, easing within a few days as the digestive system settles. It does not affect everyone, and the pattern can differ considerably between individuals.

Diarrhoea versus constipation

It is worth noting that Mounjaro can affect different people in opposite ways — some experience diarrhoea, while others find that Mounjaro causes constipation instead. Both are related to the same change in gut motility. If your bowel habits keep shifting between the two, noting each episode with dates can help your prescriber see the full picture.

A note on more serious symptoms

Severe or persistent abdominal pain — especially pain that radiates to the back — is not a typical side effect of diarrhoea and should not be dismissed. This kind of pain, particularly when accompanied by nausea and vomiting that does not resolve, can be a sign of a more serious condition affecting the pancreas or gallbladder. If you experience symptoms like this, contact your healthcare professional promptly rather than waiting for your next appointment. You can read about the broader range of Mounjaro side effects in our full guide.

One of the most practical things you can do when dealing with recurring gut symptoms is to log them consistently. GLP Trackr's side-effect tracker lets you record symptoms — including diarrhoea, nausea, and general digestive discomfort — alongside the date and time, so you can start to see whether episodes cluster around your weekly injection or a dose change.

Over time, this kind of pattern data becomes genuinely useful to bring to your prescriber. Rather than trying to recall how the past few weeks felt, you can show them a clear log. The app also lets you note mood, energy, and sleep through the daily check-in, which can reveal how gut symptoms affect the rest of your day.

If you have noticed other physical changes since starting Mounjaro — such as changes to your hair — it is worth logging those too. Some people on GLP-1 medicines report hair shedding, often linked to the physical stress of rapid weight change rather than the medicine itself. You can read more about hair loss on Mounjaro and hair loss during weight loss in our related guides.

GLP Trackr is a tracking and journaling companion — it is not a substitute for clinical advice, and it does not tell you what to do about symptoms. Any decision about managing side effects, adjusting your dose, or whether to continue treatment belongs with your prescriber or doctor, who knows your full history. If symptoms are severe, persistent, or worrying at any point, reach out to your healthcare professional rather than waiting.

Side effects worth reporting: if you experience a serious or unexpected reaction, note that your country may have an official reporting scheme — for example, the UK's Yellow Card scheme — through which patients can report suspected side effects to medicines regulators.

Ready to start logging your symptoms in one place? GLP Trackr is available on the App Store and Google Play — try it free for 7 days and see your own patterns come into focus.