Constipation is a common side effect of Mounjaro (tirzepatide), affecting approximately 5–10% of patients. This occurs because Mounjaro slows gastric emptying and reduces gut motility, which can lead to harder, less frequent stools. Whilst lifestyle measures such as increased hydration and dietary fibre are essential, many patients require laxative therapy for effective symptom relief. Choosing the best laxative for Mounjaro depends on individual symptoms, severity, and medical history. This article reviews safe laxative options, when to seek medical advice, and practical strategies to support bowel health whilst taking Mounjaro.

Why Mounjaro Can Cause Constipation

Mounjaro (tirzepatide) is a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus and for weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity. Whilst Mounjaro offers significant metabolic benefits, gastrointestinal side effects are amongst the most commonly reported adverse reactions, with constipation affecting approximately 5-10% of patients according to clinical trial data.

The mechanism underlying constipation relates to the drug's action on GIP and GLP-1 receptors throughout the gastrointestinal tract. Activation of these receptors slows gastric emptying and reduces gut motility, which helps to regulate blood glucose levels and promote satiety. However, this delayed transit time can result in harder, less frequent stools. Additionally, patients taking Mounjaro often experience reduced appetite and lower fluid and fibre intake, further compounding the risk of constipation.

Constipation typically emerges during the initial weeks of treatment or following dose escalation, and may lessen over time. Symptoms may include infrequent bowel movements (fewer than three per week), straining, hard or lumpy stools, and a sensation of incomplete evacuation. While often mild to moderate in severity, persistent constipation can significantly impact quality of life and, in rare cases, lead to complications such as faecal impaction or bowel obstruction. Understanding the pharmacological basis of this side effect is essential for implementing appropriate management strategies and ensuring patient adherence to therapy.

If you experience side effects from Mounjaro, you can report them via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Safe Laxative Options While Taking Mounjaro

When lifestyle modifications alone are insufficient to manage constipation in patients taking Mounjaro, laxatives can provide effective symptomatic relief. The choice of laxative should be guided by the severity of symptoms, patient preference, and any underlying medical conditions. There are no known direct drug interactions between tirzepatide and commonly used laxatives, but patients should always inform their GP or pharmacist of all medications they are taking.

Bulk-forming laxatives, such as ispaghula husk (Fybogel) or sterculia (Normacol), are often considered first-line options for chronic constipation. These agents work by absorbing water in the intestine, increasing stool bulk and stimulating peristalsis. They are generally well tolerated and suitable for longer-term use. However, adequate fluid intake is essential to prevent obstruction, and they may take 2-3 days to produce an effect. Note that bulk-forming laxatives should be avoided if faecal impaction is suspected.

Osmotic laxatives, including macrogol (Movicol, Laxido) and lactulose, draw water into the bowel, softening stools and facilitating passage. Macrogols typically work within 1-3 days and are recommended by NICE Clinical Knowledge Summaries as first-line treatment for faecal impaction and as a second-line option for chronic constipation when bulk-forming laxatives are ineffective. Lactulose may cause bloating and flatulence in some individuals and usually takes 2-3 days to work.

Stimulant laxatives such as senna or bisacodyl can be used when other measures have failed. These agents stimulate intestinal contractions and typically work within 6-12 hours. They should be used at the lowest effective dose, with monitoring for abdominal cramps and electrolyte disturbances. While short-term use is generally preferred, longer-term use may be appropriate in some cases under medical supervision.

Stool softeners like docusate sodium may be helpful in patients with hard, difficult-to-pass stools, though evidence for their efficacy as monotherapy is limited. Many patients benefit from a combination approach, and it is advisable to start with the gentlest effective option and escalate as necessary under medical guidance. For those with fluid restrictions (e.g., heart failure, kidney disease), fluid intake recommendations should be discussed with a healthcare professional.

When to Seek Medical Advice About Constipation

Whilst constipation is a recognised and often manageable side effect of Mounjaro, certain symptoms warrant prompt medical evaluation. Patients should contact their GP or healthcare provider if constipation persists despite appropriate laxative use and lifestyle measures for around three weeks, or if symptoms worsen over time. Persistent constipation may indicate the need for dose adjustment or alternative management strategies.

Red flag symptoms requiring urgent medical attention include severe abdominal pain, particularly if localised or accompanied by rigidity; abdominal distension or bloating that is progressive; nausea and vomiting, especially if bilious or faeculent; inability to pass stool or wind (absolute constipation); rectal bleeding or the passage of black, tarry stools (melaena); and unexplained weight loss beyond that expected from Mounjaro therapy. These features may suggest complications such as bowel obstruction, perforation, or an unrelated serious pathology requiring investigation.

Patients should also seek advice if they develop signs of faecal impaction, including overflow diarrhoea (liquid stool bypassing a hard faecal mass), severe straining, or rectal pain. Older adults and those with pre-existing bowel conditions are at higher risk of such complications. Additionally, any new or worsening symptoms following a dose increase should be reported, as dose titration may need to be slowed or temporarily paused.

It is important to remember that constipation can occasionally mask other gastrointestinal conditions. In line with NICE guidance (NG12), a change in bowel habit lasting 6 weeks or more in people aged 60 and over, or rectal bleeding in those aged 50 and over, may require further investigation. Severe, persistent abdominal pain, especially if radiating to the back and accompanied by vomiting, requires urgent assessment as this could indicate pancreatitis, a rare but serious side effect of GLP-1 receptor agonists. Patients should never hesitate to discuss concerns with their healthcare team, as early intervention can prevent complications and optimise treatment outcomes.

Lifestyle Changes to Support Bowel Health on Mounjaro

Non-pharmacological interventions form the cornerstone of managing constipation in patients taking Mounjaro and should be implemented alongside any laxative therapy. These measures not only alleviate symptoms but also support overall metabolic health and treatment adherence.

Adequate hydration is paramount. Patients should aim to drink at least 1.5 to 2 litres of water daily, as reduced fluid intake is a common contributor to constipation, particularly when appetite is suppressed. Those with conditions requiring fluid restriction (such as heart failure, kidney disease or liver disease) should discuss appropriate fluid intake with their healthcare provider. Caffeinated and alcoholic beverages should not be relied upon for hydration, as they can have a diuretic effect. Carrying a water bottle and setting regular reminders can help maintain consistent fluid intake throughout the day.

Dietary fibre intake should be gradually increased to around 30 grams per day, in line with UK guidance. Good sources include wholegrain cereals, oats, brown rice, wholemeal bread, fruits (especially prunes, pears, and apples with skin), vegetables, pulses, nuts, and seeds. Fibre increases stool bulk and promotes regular bowel movements. However, it is important to increase fibre intake slowly to avoid bloating and gas, and always in conjunction with adequate fluids. Some people, particularly those with irritable bowel syndrome, may need to adjust the types of fibre they consume based on individual tolerance.

Regular physical activity stimulates intestinal motility and can significantly improve bowel function. Patients should aim for at least 150 minutes of moderate-intensity exercise per week, as recommended by UK Chief Medical Officers' Physical Activity Guidelines. Even light activities such as walking, swimming, or gardening can be beneficial, particularly for those new to exercise or managing obesity.

Establishing a regular bowel routine is also helpful. Patients should respond promptly to the urge to defaecate and allow sufficient time for bowel movements without straining. The gastrocolic reflex is strongest after meals, so attempting to open bowels after breakfast may be particularly effective. Adopting a squatting position (or using a footstool to elevate the feet while seated on the toilet) can facilitate easier passage of stools. By combining these lifestyle strategies with appropriate medical management, most patients can successfully manage constipation whilst continuing to benefit from Mounjaro therapy.

Frequently Asked Questions