Black stools (melaena)

Truly black, sticky, tar-like stools with a strong offensive smell are called melaena, and they usually mean bleeding from high up in the digestive tract — the oesophagus, stomach or duodenum. The black colour comes from blood being chemically altered as it is digested on its way through the gut, which is why upper-gut bleeding looks black rather than red by the time it is passed. The commonest causes are a peptic ulcer (an ulcer in the stomach or duodenum, often from H. pylori infection or anti-inflammatory painkillers), inflammation of the stomach lining (gastritis), and bleeding from swollen veins in the food pipe in people with serious liver disease. Because this bleeding can be brisk and significant, melaena is treated as an important warning sign. Not everything black is blood, however: iron tablets and bismuth-containing medicines (such as some indigestion remedies), and dark foods like liquorice or blueberries, can darken the stool harmlessly — but these do not have the sticky, tarry consistency and foul smell of true melaena, and blood loss also tends to cause tiredness, breathlessness and pallor from anaemia. When in doubt, black stools should be checked, because a genuine upper-gut bleed can be serious.

• Peptic ulcer (stomach or duodenal ulcer) — a common cause; often from H. pylori or anti-inflammatory painkillers.
• Gastritis — inflammation of the stomach lining.
• Bleeding oesophageal varices — swollen veins in people with serious liver disease; a serious cause.
• Other upper-gut bleeding — such as a tear or, less often, a tumour.
• Not blood (harmless) — iron tablets, bismuth medicines, and dark foods like liquorice or blueberries, which lack the tarry, foul-smelling quality of true melaena.

When melaena is suspected, the key is to confirm bleeding and assess its impact while the source is found, usually by an urgent endoscopy. Blood tests support this: haemoglobin and a full blood count show whether enough blood has been lost to cause anaemia, and ferritin and iron studies reflect chronic slow loss. Kidney function (urea) can rise after an upper-gut bleed. A test for H. pylori is relevant when an ulcer is the cause. Because the priority in true melaena is a prompt clinical assessment rather than waiting on tests, this is a symptom to act on. Mediora.AI can surface a low haemoglobin and ferritin that fit blood loss from the upper gut, but black tarry stools need prompt medical attention.