Ulcerative colitis and black stool: what to know

Ulcerative colitis commonly affects the large bowel, where bleeding may look bright red or mixed with stool. Black, sticky, tar-like stool can instead indicate blood that has been digested higher in the digestive tract. Iron tablets, bismuth-containing products, and some foods can also darken stool. Colour alone cannot distinguish these causes, so mention any new black stool and the products you have taken. Dizziness, weakness, breathlessness, or a fast heartbeat may suggest significant blood loss.

Contact a clinician promptly and describe whether the stool is truly black and tarry, how often it has occurred, and whether you have pain, vomiting, or diarrhoea. Keep medication packets available, including iron or stomach remedies. Do not stop prescribed treatment unless advised, but do not take additional anti-inflammatory painkillers without checking, as they may irritate the digestive tract. A clinician may examine you and arrange blood or stool tests.

Seek emergency care for black tarry stool with fainting, severe weakness, confusion, breathlessness, chest discomfort, repeated vomiting, severe abdominal pain, or large amounts of blood. Urgent assessment is also needed if black stools recur or you feel increasingly unwell. Do not drive yourself if you feel faint; ask someone to help or call local emergency services.

Could a medicine or food explain the colour, or could this be digestive-tract bleeding? Do I need blood tests, stool testing, or another examination? Which pain relievers are suitable? Should my colitis treatment change while this is investigated?