Cause of extremely dark stools

Patient: My stool is extremely dark, what could be a cause of that.

Doctor: Dark stool may be caused by bleeding in the upper digestive tract (stomach and duodenum) due to conditions like duodenal or gastric ulcer, esophageal varices, gastritis or any disease that may cause bleeding like hemophilia, tuberculosis, reduced platelet counts etc. Constipation, certain medications like iron supplements, bismuth, aspirin or other NSAIDs,  or foods like black licorice or blueberries may also produce dark stools. You should consult your doctor for the identification of the cause and early treatment.