Why Mounjaro Can Cause Green Diarrhoea — and What to Do

Why does Mounjaro turn stools green?

Bile is a digestive fluid produced by the liver, stored in the gallbladder, and released into the small intestine to break down fats. Normally, as it travels through the bowel, bacteria convert it from its original bright-yellow-green colour into the brown tones most people expect. That conversion takes time.

Mounjaro acts on both GLP-1 and GIP receptors (the only dual-agonist weight-loss medicine licensed in the UK) and one consequence of that mechanism is altered gut motility. When stool moves faster than usual, bile does not have enough time to complete that colour change. The result is green or yellowish-green diarrhoea, a well-documented response to Mounjaro's effect on gut transit speed. It is not a sign that something is wrong with your liver or gallbladder, and it is not caused by the pen itself.

Diet plays a role too. People starting Mounjaro often eat less, eat differently, or consume more vegetables as part of a reduced-calorie approach. High intakes of leafy greens (spinach, kale, broccoli) can shift stool colour on their own. A combination of dietary change and faster gut transit makes green stools particularly common in the early weeks. The broader side-effect picture for Mounjaro covers how most GI changes fit into this pattern.

How long does green diarrhoea typically last on Mounjaro?

There is a widely held expectation that gut side effects disappear within a few days. That is sometimes true, but not reliably so. For many people, diarrhoea (including green diarrhoea) settles within one to two weeks of a new dose. For others it persists longer, particularly after each dose step up during the titration schedule.

Mounjaro treatment begins at 2.5mg and increases incrementally; the prescriber decides the pace and the dose. Each increase can briefly reset the gut-adjustment process. So if green diarrhoea seemed to ease off and then returned, a recent dose change is the most likely reason. The pattern of diarrhoea following a dose change is worth reading if your symptoms have returned after a period of calm.

Practical steps that often help: smaller, lower-fat meals reduce the amount of bile released at once; staying well hydrated matters more than usual when you have loose stools; eating slowly, with plenty of fibre from tolerated foods, gives the gut something to work with. None of these replace clinical advice, if the diarrhoea is frequent or is affecting your daily life, speak to your prescriber rather than adjusting things alone.

When should green diarrhoea make you pick up the phone?

Most green diarrhoea on Mounjaro is uncomfortable rather than dangerous. There are situations, though, where it is the right time to seek medical help promptly.

Contact a doctor or call NHS 111 if: diarrhoea is severe or has continued without improvement for more than a week; you are showing signs of dehydration (dark urine, dizziness, dry mouth, not passing urine); there is blood or mucus in your stool; you have a high temperature alongside the bowel symptoms; or the change in your stool came with significant, unexplained abdominal pain.

Separately, the MHRA published a Drug Safety Update in January 2026 highlighting acute pancreatitis as an infrequent but serious side effect of GLP-1 medicines. Severe stomach pain that persists and radiates to the back (with or without vomiting) is the warning sign: seek urgent medical help rather than waiting to see whether it passes. The NHS medicines information for tirzepatide, available at nhs.uk, lists the full range of side effects and the situations that need prompt attention. You can also report any side effects you are concerned about through the MHRA Yellow Card scheme.

If you are unsure whether what you're experiencing is typical or not, the team at our pharmacy offers priority aftercare seven days a week, that is part of what the service covers.

Does green diarrhoea mean Mounjaro is not working?

This is one of those worries our prescribers hear regularly, and it is worth laying to rest. Green diarrhoea has no bearing on whether tirzepatide is doing its job. The two things (gut motility changes and the appetite and metabolic effects of the medicine) are driven by overlapping but distinct processes. People who experience noticeable GI side effects are not experiencing more or less weight reduction as a result.

What green diarrhoea does tell you is that your gut is responding to the medicine, which is expected. The Mounjaro treatment overview explains how the clinical programme measured outcomes across thousands of participants, and guidance on managing more severe diarrhoea covers the practical steps available to you. If side effects are making treatment feel unmanageable, that is a conversation to have with your prescriber, dose timing and lifestyle adjustments can make a material difference for many people.

For those considering starting treatment, our prescribers discuss all of this as part of the consultation process. Start your free consultation to speak to a GPhC-registered prescriber about whether Mounjaro is clinically appropriate for you.