Why does bowel cancer cause blood in stool?

Bleeding from the lower bowel may appear as bright red blood on the stool, toilet paper, or in the toilet. Blood that has travelled further through the digestive tract can make stool look dark or black. A growth may have a fragile surface with small blood vessels that bleed on contact. Blood in Stool is a very common presentation of Bowel Cancer, but bleeding can also be caused by conditions such as haemorrhoids, anal fissures, inflammation, or polyps. The appearance alone cannot identify the cause.

Arrange a medical appointment for new, recurrent, or unexplained blood in stool, even if the amount seems small or it comes and goes. A clinician will ask about the colour and timing of bleeding, bowel-habit changes, pain, medicines, and family history. Examination and tests may be used to locate the source. Do not assume rectal bleeding is from piles without an assessment, particularly if it persists or is accompanied by other changes.

Seek urgent medical care for heavy bleeding, black tar-like stools, fainting, dizziness, severe weakness, severe abdominal pain, or shortness of breath. Contact a clinician promptly if bleeding occurs with a persistent change in bowel habit, abdominal swelling, marked tiredness, or unintentional weight loss. These warning signs need assessment regardless of the eventual cause.

Ask what may be causing the bleeding, which examination or tests are appropriate, and how to prepare. Tell the clinician about blood-thinning medicines, iron tablets, bowel screening results, and close relatives with bowel cancer or bowel polyps.