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Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist prescribed for type 2 diabetes and weight management in the UK. Whilst gastrointestinal side effects such as nausea and diarrhoea are common with Mounjaro, some patients report changes in stool colour, including green poop. These changes typically result from altered gut transit time or dietary modifications rather than a direct medication effect. Understanding when stool changes are benign and when they require medical attention helps patients manage their treatment safely and confidently.
Quick Answer: Green poop whilst taking Mounjaro typically results from rapid intestinal transit preventing complete bile breakdown, or from dietary changes, rather than a direct medication effect.
Mounjaro (tirzepatide) is a prescription medicine authorised in the UK for the treatment of type 2 diabetes mellitus and, more recently, for weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with weight-related comorbidities, as an adjunct to reduced-calorie diet and increased physical activity. It belongs to a novel class of medications known as dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonists. This dual mechanism distinguishes Mounjaro from other GLP-1 receptor agonists currently available.
The medication works by mimicking the action of two naturally occurring incretin hormones in the body. GLP-1 stimulates insulin secretion when blood glucose levels are elevated, suppresses glucagon release (which reduces glucose production by the liver), slows gastric emptying, and promotes satiety through central appetite regulation. GIP also enhances insulin secretion and may have additional effects on metabolism, though the precise mechanisms contributing to weight loss are not fully understood. By activating both pathways simultaneously, tirzepatide produces significant improvements in glycaemic control and substantial weight reduction.
Mounjaro is administered as a once-weekly subcutaneous injection, typically in the abdomen, thigh, or upper arm. The dose is gradually increased over several weeks to minimise gastrointestinal side effects, starting with 2.5 mg weekly for 4 weeks as an initiation dose, and potentially escalating to a maximum maintenance dose of 15 mg weekly, depending on individual response and tolerability. The National Institute for Health and Care Excellence (NICE) has issued guidance on its use within the NHS for type 2 diabetes.
As with all medications that affect gastrointestinal function and motility, Mounjaro can produce various digestive symptoms, including changes in bowel habits and stool characteristics. Understanding these effects helps patients distinguish between expected medication responses and symptoms requiring medical attention.
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Gastrointestinal adverse effects are the most frequently reported side effects associated with Mounjaro, particularly during the initial weeks of treatment and following dose escalations. These occur because the medication significantly slows gastric emptying and alters gut motility as part of its therapeutic mechanism. The most common digestive symptoms include:
Nausea (very common, affecting more than 1 in 10 people)
Diarrhoea (very common, affecting more than 1 in 10 people)
Decreased appetite (very common, a desired therapeutic effect but can be uncomfortable)
Vomiting (common, affecting up to 1 in 10 people)
Constipation (common, affecting up to 1 in 10 people)
Abdominal pain or discomfort (common)
Dyspepsia (indigestion)
Flatulence and bloating
Regarding stool colour changes, including green stools, there is no official direct link established in the clinical trial data or product literature for Mounjaro. However, changes in stool colour can occur for several reasons when taking medications that affect digestion. Green stools may result from:
Rapid transit time through the intestines: When Mounjaro causes diarrhoea or accelerated bowel movements, bile (which is naturally green-yellow) may not have sufficient time to break down completely, resulting in greenish stools. Bile is produced by the liver and stored in the gallbladder to aid fat digestion; it normally transforms from green to brown as it travels through the intestinal tract.
Dietary factors: Changes in appetite and food choices whilst taking Mounjaro may lead to increased consumption of green vegetables (spinach, kale, broccoli), foods with green food colouring, or iron supplements, all of which can impart a green colour to stools.
It's also worth noting that tirzepatide and similar medications have been associated with an increased risk of gallbladder problems, including gallstones, which can sometimes affect bile flow and potentially stool appearance.
Most digestive side effects, including transient stool colour changes, tend to improve within the first few weeks as the body adjusts to the medication. The gradual dose titration schedule is specifically designed to minimise these gastrointestinal symptoms.
Whilst green stools alone are generally not a cause for concern, certain accompanying symptoms warrant prompt medical evaluation. You should contact your GP or healthcare provider if you experience:
Persistent green stools lasting more than a week without obvious dietary explanation
Black, tarry stools (melaena), which may indicate upper gastrointestinal bleeding
Bright red blood in stools (haematochezia) or on toilet paper
Pale, clay-coloured, or white stools, which could suggest bile duct obstruction or liver problems
Diarrhoea lasting more than 48 hours, particularly if accompanied by dehydration symptoms
Severe abdominal pain, especially if localised to the upper abdomen and radiating to the back, which could indicate pancreatitis (a rare but serious side effect of GLP-1 receptor agonists)
Right upper quadrant pain, particularly after fatty meals, which may suggest gallbladder problems
Yellowing of the skin or eyes (jaundice) with dark urine, which could indicate liver or gallbladder issues
Signs of dehydration: excessive thirst, dark urine, dizziness, reduced urination, or dry mouth
Persistent vomiting preventing adequate fluid or medication intake
Unexplained weight loss beyond expected therapeutic effects
Fever accompanying stool changes, suggesting possible infection
For urgent but non-emergency concerns, contact NHS 111. Seek immediate medical attention (call 999 or attend A&E) if you develop:
Severe, unrelenting abdominal pain
Persistent vomiting with inability to keep down fluids
Signs of severe dehydration (confusion, rapid heartbeat, very dark urine, no urination for 8+ hours)
Symptoms of allergic reaction (difficulty breathing, facial swelling, severe rash)
It is important to note that pancreatitis, though uncommon, is a recognised risk with GLP-1 receptor agonists. Symptoms include severe upper abdominal pain (often radiating to the back), nausea, and vomiting. If you suspect pancreatitis, stop taking Mounjaro immediately and seek urgent medical care.
If you are taking insulin or sulfonylureas alongside Mounjaro and experience digestive symptoms that prevent normal eating, monitor your blood glucose more frequently and seek advice about temporary medication adjustments to prevent hypoglycaemia.
For routine concerns about stool changes or digestive symptoms, schedule an appointment with your GP or diabetes specialist nurse. They can assess whether symptoms are medication-related or require investigation for other gastrointestinal conditions. Never discontinue Mounjaro without medical guidance, as abrupt cessation may affect your diabetes control or weight management progress.
Several evidence-based strategies can help minimise gastrointestinal side effects whilst taking Mounjaro, improving both comfort and medication adherence. Dietary modifications are often the first-line approach:
Eat smaller, more frequent meals rather than large portions, as this reduces the burden on your slowed digestive system. Aim for five to six small meals throughout the day rather than three large ones.
Choose easily digestible foods, particularly during the first weeks after starting Mounjaro or increasing your dose. Focus on:
Lean proteins (chicken, fish, tofu)
Well-cooked vegetables
White rice, pasta, or potatoes
Bananas and other low-fibre fruits
Plain yoghurt
Avoid or limit foods that may exacerbate symptoms:
High-fat, greasy, or fried foods (which slow digestion further)
Spicy dishes
Carbonated beverages
Caffeine and alcohol
Foods high in artificial sweeteners
Stay well hydrated by drinking regularly throughout the day, aiming for 6-8 glasses of fluid daily. Sip water frequently rather than consuming large amounts at once. If you have heart or kidney disease with fluid restrictions, consult your healthcare provider about appropriate hydration targets.
Timing considerations: Take Mounjaro at a consistent time each week. Some patients find that administering the injection at different times of day (such as evenings) may help manage side effects, though this is based on individual preference. Eat slowly and chew food thoroughly to aid digestion.
Medication strategies may include:
Anti-nausea medications if prescribed by your doctor for severe nausea. Note that domperidone has cardiac risks (including QT prolongation) and is restricted to short-term use, while metoclopramide carries a risk of movement disorders and should only be used for short courses.
Loperamide for diarrhoea management after consulting your healthcare provider. Do not use loperamide if you have fever, blood in your stool, or suspected infection.
Be aware that Mounjaro's effect on gastric emptying may affect the absorption of other medications. This is particularly important for oral contraceptives—consider using non-oral contraception or additional barrier methods for 4 weeks after starting Mounjaro and after each dose increase.
Lifestyle measures:
Avoid lying down immediately after eating (wait at least 2-3 hours)
Gentle physical activity, such as short walks after meals, can aid digestion
Stress management techniques, as anxiety can worsen gastrointestinal symptoms
Monitoring and communication: Keep a symptom diary noting when digestive issues occur, their severity, and any dietary or lifestyle factors that seem to help or worsen symptoms. Share this information with your healthcare team, as they may adjust your dose escalation schedule or provide additional management strategies.
Most patients find that gastrointestinal side effects significantly improve after the first 4-8 weeks of treatment. If symptoms remain troublesome despite these measures, discuss with your prescriber whether a slower dose titration or alternative treatment approach might be appropriate. The benefits of improved glycaemic control and weight management should be balanced against tolerability.
If you experience any suspected adverse reactions to Mounjaro, you or your healthcare professional can report them via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.
Green poop is not officially listed as a direct side effect of Mounjaro, but it can occur due to rapid intestinal transit during diarrhoea (a very common side effect) or dietary changes. Most cases are temporary and resolve as your body adjusts to the medication.
Contact your GP if green stools persist beyond one week without dietary explanation, or immediately if you notice black tarry stools, bright red blood, pale clay-coloured stools, severe abdominal pain, or signs of dehydration. These may indicate conditions requiring urgent assessment.
Eat smaller, more frequent meals with easily digestible foods, avoid high-fat and spicy dishes, stay well hydrated, and allow 2-3 hours after eating before lying down. Most gastrointestinal symptoms improve significantly within 4-8 weeks of starting treatment.
All medical content on this blog is created based on reputable, evidence-based sources and reviewed regularly for accuracy and relevance. While we strive to keep content up to date with the latest research and clinical guidelines, it is intended for general informational purposes only.
DisclaimerThis content is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional with any medical questions or concerns. Use of the information is at your own risk, and we are not responsible for any consequences resulting from its use.
