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Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist authorised in the UK for type 2 diabetes and weight management. Whilst gastrointestinal side effects such as nausea, constipation, and abdominal discomfort are common with Mounjaro, concerns about whether Mounjaro causes bowel obstruction have emerged. Although bowel obstruction is not listed as an established side effect, rare post-marketing reports of ileus and intestinal obstruction with GLP-1-based therapies warrant awareness. This article examines the evidence, warning signs, and when to seek medical advice whilst taking Mounjaro.
Quick Answer: Bowel obstruction is not an established side effect of Mounjaro, though rare post-marketing reports of ileus and intestinal obstruction have been identified with GLP-1-based therapies.
Mounjaro (tirzepatide) is a prescription medicine authorised in the UK for the treatment of type 2 diabetes mellitus and, as of 2023, for weight management in adults with obesity or overweight with weight-related comorbidities. 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 such as semaglutide (Ozempic, Wegovy) or liraglutide (Victoza, Saxenda).
The medication works by mimicking the action of two naturally occurring incretin hormones. GLP-1 stimulates insulin secretion when blood glucose levels are elevated, suppresses glucagon release, slows gastric emptying, and reduces appetite through effects on the central nervous system. GIP also enhances insulin secretion and may influence fat metabolism and energy expenditure. Together, these actions help improve glycaemic control in people with type 2 diabetes and promote weight loss by reducing caloric intake and delaying stomach emptying.
Mounjaro is administered as a once-weekly subcutaneous injection, typically starting at a low dose (2.5 mg) and gradually increasing over several weeks to minimise gastrointestinal side effects. According to the Summary of Product Characteristics (SmPC), the maintenance dose varies depending on individual response and tolerability, ranging from 5 mg to a maximum of 15 mg weekly. Mounjaro is not indicated for type 1 diabetes and has not been studied in patients with severe gastrointestinal disease, including severe gastroparesis. As with all GLP-1-based therapies, the slowing of gastric emptying is a key pharmacological effect but also contributes to some of the gastrointestinal adverse effects patients may experience.
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Gastrointestinal adverse effects are among the most commonly reported side effects of Mounjaro, occurring in a significant proportion of patients, particularly during dose initiation and escalation. According to the SmPC, nausea is classified as 'very common' (affecting ≥1/10 patients), while vomiting, diarrhoea, constipation, abdominal pain, and dyspepsia are 'common' (affecting ≥1/100 to <1/10 patients). These effects are largely attributable to the medication's mechanism of slowing gastric emptying and altering gut motility.
Constipation is particularly relevant when considering bowel-related concerns. The delayed gastric emptying and reduced gut motility can lead to harder, less frequent stools. While usually mild to moderate in severity, persistent or severe constipation requires attention, as it can progress to faecal impaction or, in rare circumstances, contribute to more serious complications. Management includes adequate hydration, gradual increase in dietary fibre, regular physical activity, and if needed, osmotic laxatives (following healthcare professional advice).
Regarding bowel obstruction specifically, while there is no established direct causal relationship in the product literature, post-marketing surveillance for GLP-1-based therapies has identified rare reports of ileus (a form of bowel obstruction where the intestine stops moving properly) and intestinal obstruction. The MHRA and EMA continue to monitor these events through pharmacovigilance. It remains unclear whether these events are directly caused by the medication or occur coincidentally in a population already at risk due to diabetes, obesity, or other comorbidities.
Patients should be aware that severe gastrointestinal side effects may lead to dehydration, which in some cases could affect kidney function. If experiencing persistent vomiting, diarrhoea or severe constipation, patients should seek medical advice promptly. If you suspect an adverse reaction to Mounjaro, this can be reported through the MHRA Yellow Card scheme.
Bowel obstruction is a serious medical condition in which the normal passage of intestinal contents is blocked, either partially or completely. While not a commonly recognised side effect of Mounjaro, it is important for patients and healthcare professionals to recognise the warning signs, particularly given the medication's effects on gastrointestinal motility.
Key symptoms of bowel obstruction include:
Severe abdominal pain or cramping that is persistent and worsening, often described as colicky (coming in waves)
Abdominal distension or bloating that is progressive and accompanied by a visibly swollen abdomen
Nausea and vomiting, particularly if vomiting becomes frequent, bilious (green), or faeculent (foul-smelling, suggesting bowel content)
Inability to pass wind or stools (absolute constipation), which may indicate complete obstruction
High-pitched bowel sounds on examination, or conversely, absent bowel sounds in advanced cases
Dehydration symptoms such as reduced urine output, dizziness, or dry mouth
It is crucial to distinguish between common constipation—which may cause mild discomfort and infrequent bowel movements—and signs of obstruction, which are typically more severe, acute in onset, and associated with systemic symptoms. Patients taking Mounjaro who experience worsening constipation that does not respond to usual measures should seek medical review. Importantly, do not self-treat suspected bowel obstruction with laxatives, as these may worsen the situation in cases of actual obstruction.
Certain individuals may be at higher risk for bowel complications, including those with a history of abdominal surgery, inflammatory bowel disease, hernias, or previous episodes of bowel obstruction. Older adults and those with significant comorbidities should be particularly vigilant. If any combination of the above symptoms occurs, especially severe or persistent abdominal pain with vomiting and inability to pass stools, urgent medical assessment is required.
Patients prescribed Mounjaro should be counselled on when to contact their GP, diabetes specialist nurse, or seek emergency care. While most gastrointestinal side effects are mild and self-limiting, certain symptoms warrant prompt medical evaluation.
Contact your GP or diabetes care team if you experience:
Persistent nausea or vomiting lasting more than 48 hours, or vomiting that prevents you from keeping down fluids or medications
Severe or worsening constipation that does not improve with over-the-counter laxatives or dietary measures within a few days
Moderate abdominal pain that is new, persistent, or progressively worsening
Signs of dehydration, including reduced urination, dizziness, dry mouth, or feeling faint
Unexplained weight loss beyond expected therapeutic effect, or inability to maintain adequate nutrition
Seek urgent medical attention (A&E or call 999) if you develop:
Severe, unrelenting abdominal pain, particularly if accompanied by a rigid, distended abdomen
Persistent vomiting with inability to tolerate any oral intake
Complete inability to pass stools or wind for more than 24 hours, especially with abdominal distension
Vomiting blood or passing black, tarry stools (melaena), which may indicate gastrointestinal bleeding
Fever with abdominal pain, which could suggest perforation or infection
In England, NHS 111 can provide urgent non-emergency advice if you're unsure about the severity of your symptoms.
It is important to be aware that Mounjaro can reduce the exposure to oral contraceptives. If you use oral contraceptives, additional contraceptive methods are recommended for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase, as stated in the SmPC.
If you are scheduled for surgery or endoscopic procedures, inform your surgical team that you are taking Mounjaro. Current UK perioperative guidance from the Centre for Perioperative Care and Association of Anaesthetists recommends an individualised approach; your healthcare team may advise on diet modification, temporary withholding of the medication, or other precautions based on your specific procedure and risk factors.
Regular follow-up with your prescribing clinician is essential to monitor treatment response, adjust dosing, and address any emerging side effects. NICE guidance emphasises shared decision-making and ongoing review for patients on GLP-1-based therapies. Never stop Mounjaro abruptly without medical advice, but do report any concerning symptoms promptly to ensure safe and effective treatment.
No, bowel obstruction is not a commonly recognised side effect of Mounjaro. However, rare post-marketing reports of ileus and intestinal obstruction have been identified with GLP-1-based therapies, and regulatory authorities continue to monitor these events.
Common gastrointestinal side effects of Mounjaro include nausea (very common), constipation, vomiting, diarrhoea, abdominal pain, and dyspepsia. These effects are largely due to delayed gastric emptying and altered gut motility.
Seek urgent medical attention if you experience severe unrelenting abdominal pain, persistent vomiting with inability to tolerate fluids, complete inability to pass stools or wind for over 24 hours, vomiting blood, passing black tarry stools, or fever with abdominal pain.
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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.
