Blood seen in the stool can originate anywhere along the intestinal tract.
A black stool usually means that the blood is coming from the upper part of the gastrointestinal (GI) tract. At least 6 tablespoons (or 200 milliliters) of blood must have been lost in order to cause passage of melena. Passing maroon-colored stools or bright red blood (hematochezia) usually suggests that the blood is coming from the large bowel or the rectum. However, sometimes hematochezia can be caused by massive upper GI bleeding.
The ingestion of black licorice, lead, iron pills, Pepto-Bismol (or similar medications containing bismuth subsalicylate) or blueberries can all cause black stools, or false melena. If a problem is suspected, stools should be tested for the presence of hidden blood.
Some upper GI causes of bloody stools, such as peptic ulcer disease, can also cause vomiting blood.
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