Independent UK medicine guidance
Mounjaro and bowel obstruction (ileus): the warning signs that need A&E
Ileus means the bowel has stopped moving. It is rare, it is serious, and it has been reported with medicines in the GLP-1 class, including tirzepatide, since they came into wide use. Constipation and bloating on Mounjaro are common and usually harmless; ileus and mechanical obstruction are not, and the difference is in the pattern of symptoms. This guide is short on purpose: it tells you what the warning signs are, what to do, and how ordinary constipation differs.
Key facts
- Ileus is not listed as an adverse reaction in the current UK Summary of Product Characteristics for tirzepatide. What is listed, as uncommon, is delayed gastric emptying.[2] Ileus does appear in the US and Canadian labelling for tirzepatide and has been reported with GLP-1 medicines in post-marketing use.
- The leaflet’s own urgent line covers it: stop Mounjaro and seek urgent medical help for severe, persistent pain in the stomach area, with or without vomiting.[1]
- Constipation is very common on Mounjaro and is not ileus.[1] The difference is pain, vomiting, a swollen abdomen and no wind.
- Previous bowel surgery, hernias, Crohn’s disease, and severe diverticular disease raise the background risk of obstruction. Tell your prescriber if any apply.
- Suspected side effects should be reported to the MHRA Yellow Card scheme.[3]
Ileus, obstruction and constipation: what the words mean
Constipation is stools that are hard, infrequent or difficult to pass. The bowel is working, just slowly. Very common on tirzepatide because the gut is slowed, intake is low and fluid is low.[1]
Ileus (sometimes called paralytic ileus) is when the bowel stops contracting altogether, without anything physically blocking it. Contents back up, the abdomen swells, and vomiting follows. It is a recognised complication after abdominal surgery and with certain medicines.
Mechanical obstruction is a physical blockage: a twist, a hernia, scar tissue from previous surgery, or a mass. The symptoms overlap with ileus and both need hospital assessment.
You cannot tell ileus from obstruction at home, and you do not need to. The same symptom pattern sends you to A&E for either.
The pattern that matters
| Ordinary Mounjaro constipation | Possible ileus or obstruction |
|---|---|
| Infrequent, hard stools; some bloating | No stools and no wind, with a swelling abdomen |
| Mild cramping that eases after a bowel movement or passing wind | Severe or constant pain, or waves of cramping that build over hours |
| No vomiting, or the occasional nausea you already had | Repeated vomiting, especially dark green or brown fluid |
| Abdomen soft to press | Abdomen tight, hard or very tender to press |
| Improves with fibre, fluid and time | Getting worse over hours despite everything |
What to do
If you have the right-hand column: do not eat or drink, do not take a laxative, do not take your next dose, and go to A&E or call 999. Laxatives against an obstruction can make things worse, which is why this is not a pharmacy problem.
If you have the left-hand column, the constipation relief guide and fibre guide are the right place, and a pharmacist can advise if a week of fibre and fluid has not worked.
Who is at higher risk
Anyone with previous abdominal surgery (adhesions), a hernia, Crohn’s disease, severe diverticular disease, or a history of obstruction has a higher background risk of a blocked bowel regardless of medicine. Tirzepatide slows the gut on top of that. The SmPC notes that the medicine has not been studied in severe gastrointestinal disease and is not recommended in it.[2] If any of this applies to you and your online questionnaire did not ask, tell your prescriber now.
What is known about the link
The UK Summary of Product Characteristics for tirzepatide does not list ileus among its adverse reactions. It does list delayed gastric emptying as uncommon, and warns separately that tirzepatide has not been studied in people with severe gastrointestinal disease.[2] The US and Canadian labelling for the same medicine goes further and names ileus and intestinal obstruction, added after reports in routine use once very large numbers of people were taking these medicines.[5] Regulators in different countries reach different conclusions about when a post-marketing report is strong enough to add to a label, and the UK has not made that addition for tirzepatide.
None of that changes what you should do. Constipation on tirzepatide is common and manageable; a bowel that has stopped is an emergency whatever the label says, and severe or persistent stomach pain is already the leaflet’s own reason to stop and seek urgent help.[1] If you experience anything like this, report it through the Yellow Card scheme so the UK picture becomes clearer.[3]
Common questions
Can Mounjaro cause a bowel obstruction?
Ileus, where the bowel stops moving, is not listed in the current UK product information for tirzepatide, though delayed gastric emptying is listed as uncommon. Ileus and intestinal obstruction do appear in the US and Canadian labelling for the same medicine, from post-marketing reports. It is rare either way, and ordinary constipation is not the same thing.
How do I know if my constipation is serious?
Constipation becomes an emergency when it comes with severe or worsening stomach pain, a swollen hard abdomen, repeated vomiting, or no wind at all. Any of those means A&E, not a laxative.
Should I stop Mounjaro if I have not had a bowel movement for a week?
A week without a bowel movement and no other symptoms is a same-day pharmacy or 111 conversation, and worth telling your prescriber. With pain, vomiting or a swollen abdomen, go to A&E. Do not take your next dose until you have been assessed.
Can I take Mounjaro after bowel surgery?
Tell your prescriber about any abdominal surgery before starting. Adhesions from previous surgery increase the risk of obstruction, and the medicine is not recommended in severe gastrointestinal disease. It is their decision, made with the facts.
Related guides
About this page. Monj is an independent information and price comparison service run by Medstack Ltd. It is not a pharmacy and does not prescribe, dispense or sell medicines. This page provides general information, not diagnosis or individual medical advice. It has not been clinically reviewed; a named clinical reviewer will be added when one has approved it. Read our editorial and review policy and independence statement.
References
- Eli Lilly. Mounjaro KwikPen: Package leaflet, information for the patient (UK), revised August 2026, section 4. electronic medicines compendium. medicines.org.uk/emc/product/15481/pil. Accessed 6 September 2026.
- Eli Lilly. Mounjaro KwikPen: Summary of Product Characteristics (UK), sections 4.4 and 4.8. electronic medicines compendium. medicines.org.uk/emc/product/15481/smpc. Accessed 6 September 2026.
- MHRA. Yellow Card scheme. yellowcard.mhra.gov.uk. Accessed 6 September 2026.
- NHS. Stomach ache. nhs.uk/conditions/stomach-ache. Accessed 6 September 2026.
- Eli Lilly. Mounjaro (tirzepatide) US prescribing information, post-marketing experience: gastrointestinal disorders including ileus. DailyMed. dailymed.nlm.nih.gov. Accessed 6 September 2026.
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