Green stools whilst taking Mounjaro (tirzepatide) can understandably cause concern, but this colour change is typically benign and temporary. Mounjaro, a dual GLP-1 and GIP receptor agonist licensed in the UK for type 2 diabetes mellitus, commonly affects gastrointestinal function during initial therapy. When digestive transit speeds up—often due to diarrhoea or dietary changes—bile pigments retain their natural green colour rather than converting to the usual brown. Whilst green poop on Mounjaro is not listed as a specific adverse effect, it reflects normal physiological responses to medication-induced changes in gut motility and is rarely a sign of serious pathology.

Understanding Green Stools While Taking Mounjaro

Mounjaro (tirzepatide) is a glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus. As with many medications that affect the gastrointestinal system, patients may notice changes in their bowel habits, including alterations in stool colour.

Green stools whilst taking Mounjaro are not specifically listed as an adverse effect in the medicine's official information but may occur as a result of other gastrointestinal effects such as diarrhoea or dietary factors. The colour of faeces is primarily determined by bile pigments—specifically bilirubin and its metabolites—which undergo chemical transformation as they pass through the intestinal tract. When food moves through the digestive system more rapidly than usual (as can happen with diarrhoea), there is insufficient time for bile to break down completely from its natural green colour to the typical brown shade.

Whilst green stools can understandably cause concern, they are usually benign and temporary, particularly when associated with the gastrointestinal adjustments that occur during the initial weeks of Mounjaro therapy. However, it is important to distinguish between harmless colour changes and those that may indicate a more significant underlying issue. Patients should be reassured that isolated episodes of green stools, in the absence of other worrying symptoms, rarely signify serious pathology. Understanding the mechanism behind this colour change can help alleviate anxiety and enable patients to monitor their symptoms appropriately whilst continuing their prescribed treatment regimen.

Why Mounjaro Can Cause Changes in Stool Colour

Mounjaro exerts its therapeutic effects by activating both GLP-1 and GIP receptors, which are found throughout the gastrointestinal tract. This dual agonist action influences several physiological processes that can directly impact bowel function and stool characteristics. The medication slows gastric emptying, which is one of its key mechanisms for improving glycaemic control and promoting satiety.

Whilst Mounjaro typically slows gastric emptying, some patients experience diarrhoea as a side effect, which can reduce intestinal transit time. When stool passes through the intestines more quickly, bile pigments do not undergo their usual transformation. Bile is secreted by the liver as a greenish-yellow fluid containing bilirubin. As it travels through the small and large intestines, bacterial enzymes convert bilirubin into stercobilin and urobilin, which give faeces their characteristic brown colour. Rapid transit prevents this conversion, resulting in green-tinged stools.

Additionally, dietary factors can contribute to this effect. Patients taking Mounjaro often make dietary modifications, sometimes increasing their intake of leafy green vegetables, which contain chlorophyll, or consuming foods with green food colouring. Iron supplements can also contribute to darker or greenish-black stools, though these are not routinely prescribed with Mounjaro unless clinically indicated. The interplay between medication-induced changes in gut function, dietary adjustments, and individual variation in gut microbiota composition means that stool colour changes are multifactorial. Green stools are not listed as a specific adverse reaction in the Mounjaro product information, and the phenomenon is generally considered a benign effect related to changes in gastrointestinal function or diet.

Common Gastrointestinal Side Effects of Mounjaro

Gastrointestinal adverse effects are the most frequently reported side effects associated with Mounjaro therapy, particularly during dose initiation and escalation. According to the MHRA product information, the most common gastrointestinal symptoms include nausea, diarrhoea (both very common, affecting more than 1 in 10 people), vomiting, constipation, abdominal pain, and dyspepsia (all common, affecting up to 1 in 10 people). Decreased appetite is also very common. These effects are generally mild to moderate in severity and tend to diminish over time as the body adapts to the medication.

Nausea is most pronounced during the first few weeks of treatment or following dose increases. This can be managed through dietary modifications, such as eating smaller, more frequent meals and avoiding high-fat or spicy foods. Diarrhoea may contribute to the rapid intestinal transit that causes green stools. Conversely, some patients experience constipation, reflecting the variable effects of GLP-1 receptor agonists on different individuals' gastrointestinal motility.

Other reported effects include bloating, flatulence, and gastro-oesophageal reflux symptoms. The MHRA's product information for Mounjaro emphasises the importance of gradual dose titration to minimise these gastrointestinal side effects. There is also a risk of hypoglycaemia when Mounjaro is used in combination with insulin or sulfonylureas, and dose adjustments of these medications may be needed.

Patients should be aware that GLP-1 receptor agonists like Mounjaro have been associated with gallbladder problems, particularly in those experiencing significant weight loss. Patients should be counselled about these potential symptoms before starting treatment and reassured that most gastrointestinal effects are transient. Maintaining adequate hydration is particularly important for those experiencing diarrhoea or vomiting, as dehydration can exacerbate symptoms and lead to additional complications. GPs should regularly review gastrointestinal tolerability during follow-up appointments and consider dose adjustment or symptomatic management strategies if side effects significantly impact quality of life or treatment adherence.

When Green Stools Require Medical Attention

Whilst green stools are usually benign in patients taking Mounjaro, certain accompanying symptoms warrant prompt medical evaluation. Patients should contact their GP or call NHS 111 if green stools are accompanied by:

• Severe or persistent abdominal pain, particularly if localised to the right upper quadrant, which could indicate biliary or hepatic pathology

• High fever (temperature above 38°C), which may suggest infection

• Blood in the stools (either bright red or dark, tarry stools), indicating possible gastrointestinal bleeding

• Persistent vomiting that prevents adequate oral intake or medication administration

• Signs of dehydration, including reduced urine output, dizziness, dry mouth, or excessive thirst

• Unexplained weight loss beyond expected therapeutic effects

• Jaundice (yellowing of the skin or eyes), which could indicate liver dysfunction

Attend A&E or call 999 immediately for:

• Severe, persistent abdominal pain, especially if accompanied by fever and vomiting

• Severe epigastric pain radiating to the back (possible pancreatitis)

• Black or tarry stools with dizziness or feeling faint

• Signs of severe dehydration or shock

Additionally, if green stools persist for more than a week despite dietary modifications, or if they are accompanied by significant changes in bowel habit (such as sudden onset of severe diarrhoea or complete constipation), medical review is advisable.

Patients with a history of inflammatory bowel disease, irritable bowel syndrome, or previous gastrointestinal surgery may be at higher risk of experiencing more pronounced digestive symptoms and should maintain closer contact with their healthcare team. It is also important to note that Mounjaro has been associated with more serious gastrointestinal complications such as pancreatitis and gallbladder disease in susceptible individuals. Any severe, persistent, or worsening abdominal symptoms should be evaluated promptly to exclude these more serious conditions and ensure patient safety.

Patients are encouraged to report any suspected side effects to the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk.

Managing Digestive Changes on Mounjaro

Effective management of digestive changes, including green stools, whilst taking Mounjaro involves a combination of lifestyle modifications, dietary adjustments, and appropriate medical monitoring. Patients can take several practical steps to minimise gastrointestinal side effects:

Dietary strategies are fundamental. Eating smaller, more frequent meals rather than large portions can reduce the burden on the digestive system. Avoiding high-fat, greasy, or heavily spiced foods may help minimise nausea and diarrhoea. Increasing soluble fibre intake gradually (through foods such as oats, bananas, and cooked vegetables) can help regulate bowel movements, though patients should be cautious not to increase fibre too rapidly, as this may worsen bloating. Adequate hydration is essential—aim for at least 1.5–2 litres of water daily, particularly if experiencing loose stools. Limiting alcohol and caffeine can also help reduce gastrointestinal irritation.

Medication timing can also make a difference. Taking Mounjaro at a consistent time each week, as prescribed, is important for effectiveness. Some patients find that administering the injection in the evening may be more practical, though this is anecdotal and not specifically recommended in the product information. Dose escalation should follow the prescribed schedule, as rushing to higher doses increases the likelihood of side effects. If side effects are troublesome, discuss with your GP about potentially slowing the titration schedule.

Keeping a symptom diary can help identify patterns and triggers, enabling patients and healthcare professionals to make informed adjustments. Recording food intake, bowel movements, and any associated symptoms provides valuable information for clinical review. If gastrointestinal symptoms are significantly impacting quality of life, GPs may consider temporarily reducing the dose or extending the interval between dose increases.

For persistent symptoms, discuss with your GP or pharmacist whether over-the-counter remedies might be appropriate. For example, loperamide may help with diarrhoea, though it should be used cautiously and only after professional advice.

Probiotics are sometimes suggested to support gut health, though evidence for their efficacy in managing GLP-1 receptor agonist-related side effects is limited. Patients should discuss any over-the-counter supplements with their GP or pharmacist before use. Regular follow-up appointments are essential to monitor treatment response, assess tolerability, and make necessary adjustments to optimise both therapeutic outcomes and patient comfort throughout the treatment journey.

Frequently Asked Questions