Diarrhoea on Mounjaro: Why It Happens and What to Keep Track Of

Mounjaro acts on two receptors — GLP-1 and GIP — that are involved in regulating appetite, digestion, and how quickly food moves through the gut. One of its commonly reported effects is a slowing of stomach emptying (gastroparesis-like slowing), and changes to gut motility more broadly. For some people this leads to constipation; for others it tips the other way and causes looser stools or diarrhoea. Both patterns are recognised, and you can read more about the other side of this on our page covering Mounjaro and constipation.

When does it typically occur?

Diarrhoea is most commonly reported in the early weeks of treatment or shortly after a dose increase. It often eases as the body adjusts, though timing varies from person to person. It is worth noting that other GLP-1-based medicines — including Wegovy (semaglutide) — carry a similar gastrointestinal profile, so this is not unique to Mounjaro specifically.

Things that may influence symptoms

• Eating large or high-fat meals, which can be harder to digest when gastric emptying is slower
• Eating too quickly or skipping smaller, more frequent meals
• Dehydration, which can make loose stools feel worse — keeping fluid intake steady is generally sensible
• Other medicines or supplements taken alongside Mounjaro — worth checking with your prescriber

For a broader overview of digestive and other reactions, see our guide to Mounjaro side effects. If you have also noticed skin changes since starting treatment, our page on Mounjaro and skin rash covers that separately.

According to the NHS, diarrhoea that is severe, contains blood, or is accompanied by significant abdominal pain warrants prompt medical attention — do not wait to see if it passes on its own in those circumstances.

One of the most useful things you can do when a side effect like diarrhoea is recurring is to build a clear, timestamped picture of it — when it happens, how severe it feels, and whether it clusters around injection day or dose changes. That is exactly what GLP Trackr's side-effect tracking is designed for.

Log each episode directly in the app, rate its severity, and add any notes about what you had eaten or how you were feeling that day. Over time, the app's insights can help you spot patterns — for example, whether symptoms are consistently worse in the 48 hours after your weekly injection, or whether they have been gradually improving week on week. That kind of structured record is far more useful to bring to a prescriber appointment than trying to recall events from memory.

Alongside side-effect logging, the food log (including the AI photo scan) can help you notice whether certain meals seem to coincide with worse symptoms — useful context without requiring you to make any dietary changes without guidance. You can also use the daily check-in to note your energy and overall wellbeing, giving a fuller picture of how treatment is going.

For more detail on the full range of what people report while on Mounjaro, visit our in-depth page on Mounjaro diarrhoea.

Important: if diarrhoea is severe, persistent, or leaving you feeling unwell, contact your healthcare professional promptly. GLP Trackr helps you record what you are experiencing so you can discuss it with your prescriber — it is not a substitute for their clinical judgement, and any decisions about your dose or treatment plan belong with them.

Start building your symptom record today — GLP Trackr is free to try for 7 days on the App Store and Google Play.