What should you know about black mucus in stool?

Mucus is a slippery substance made by the bowel. It can look dark when mixed with stool, food residue, or blood. Black stool that is sticky, tar-like, unusually foul-smelling, or repeatedly present deserves prompt medical attention because digested blood can turn black as it travels through the gut.

Iron supplements, bismuth-containing medicines, and some very dark foods can darken bowel movements without causing bleeding. A small amount of clear or pale mucus can also occur with bowel irritation. The appearance alone cannot identify the cause.

Black, tarry stool is a common presentation of bleeding from the upper digestive tract. Possible sources include inflammation of the stomach lining, Helicobacter pylori infection, or a peptic ulcer. Gastritis and peptic ulcer disease can also cause upper abdominal discomfort, nausea, or indigestion, although some people have few warning symptoms before bleeding occurs.

If the change happened once and you recently started iron or a bismuth medicine, note the timing and monitor the next bowel movements. Do not stop a prescribed medicine without checking with the prescriber. Avoid taking ibuprofen, naproxen, or aspirin for this symptom unless a clinician has told you to use them, as these medicines can contribute to stomach bleeding.

Arrange a medical assessment if black stool continues, returns, or cannot be explained by a medicine or food. A clinician may ask about medicines, abdominal symptoms, and bleeding, then decide whether stool or blood tests and further digestive-tract assessment are needed.

Seek urgent medical help now for black tarry stool with dizziness, fainting, marked weakness, confusion, breathlessness, a fast heartbeat, vomiting blood, or severe abdominal pain. These features can indicate significant digestive-tract bleeding or a complication that needs prompt treatment.

Do not wait for the stool colour to change back if you feel unwell. Tell the clinician whether the stool was sticky or tar-like, when it began, and which medicines or supplements you take. If you are taking a blood thinner or have had previous ulcers or digestive bleeding, seek advice promptly even when symptoms seem mild.

You could ask: Could my medicine or supplement explain the colour? Does the texture suggest digested blood? Do I need testing for gastritis, Helicobacter pylori infection, or a peptic ulcer? Which pain medicines should I avoid, and what symptoms mean I should seek emergency care?