Why does ulcerative colitis cause blood in stool?

In ulcerative colitis, ongoing inflammation can create small raw areas or ulcers on the surface of the colon. Stool moving across this sensitive tissue can disturb tiny blood vessels, allowing fresh red blood or blood mixed with mucus to appear. Blood in Stool is a common presentation of Ulcerative Colitis, but bleeding from the rectum or bowel also has other causes that need proper assessment.

Bleeding can occur with diarrhoea, urgency, cramps or fatigue. The appearance alone cannot identify its source or seriousness, so tell your clinician about any new bleeding or a clear change from your usual pattern.

Contact your inflammatory bowel disease team or clinician when you notice blood in stool. They may review your symptoms, medicines and recent illness, and decide whether testing or a treatment adjustment is needed. Keep hydrated, take prescribed medicines as directed, and note whether the blood is on the paper, in the bowl or mixed through the stool.

Avoid assuming that bleeding is part of your usual symptoms. A medication review is especially useful if you have missed doses, started a new medicine, or have also developed fever or worsening diarrhoea.

Seek urgent medical care if bleeding is heavy, you pass black or tar-like stool, feel faint, have chest discomfort, are short of breath, or develop severe abdominal pain. Urgent assessment is also needed for rapid weakness, repeated vomiting, a swollen abdomen or inability to drink enough fluid. These symptoms can require treatment without delay.

Ask what may be driving the bleeding, whether your ulcerative colitis treatment needs review, and how to recognise a flare that needs urgent help. Ask whether you need blood tests to check for effects of ongoing blood loss.