Black diarrhea is a concerning symptom that often signals the presence of blood in the stool, a condition called melena, which requires prompt attention. When this loose, watery stool is black, it generally means bleeding is occurring high up in the digestive tract, such as the stomach or small intestine. The dark appearance is a sign that the blood has been chemically altered by digestive juices and enzymes during its journey. Although black stool can sometimes be caused by dietary factors or medication, any sudden change to a black, loose stool should be taken seriously as it may indicate a severe medical issue.
Identifying True Melena
The medical term for black, tarry stool resulting from internal bleeding is melena. True melena is typically described as jet black, sticky, and tarry, often having a consistency similar to roofing tar. A distinguishing feature is its extremely foul and pungent odor, which is much stronger than usual stool smell. This powerful scent comes from the breakdown products of blood metabolized by intestinal bacteria. For this discoloration to occur, the bleeding source must be high enough in the digestive tract—usually the esophagus, stomach, or duodenum—to allow sufficient time for digestion.
Serious Medical Causes
The most frequent cause of upper gastrointestinal bleeding that leads to melena is peptic ulcer disease. These ulcers are open sores on the stomach or small intestine lining that can erode into blood vessels, often caused by long-term NSAID use or Helicobacter pylori infection. Another serious cause involves esophageal varices, which are enlarged veins in the lower esophagus common in people with advanced liver disease, such as cirrhosis, that can rupture suddenly. A Mallory-Weiss tear, usually caused by forceful vomiting, is a less common cause involving a tear in the esophageal mucous membrane. Malignancies in the upper gastrointestinal tract can also cause chronic or intermittent bleeding that manifests as melena.
Dietary and Medication-Related Causes
Not all black stool signals internal bleeding; ingested substances can harmlessly stain the stool a dark color, leading to “false melena.” The most common medication to cause this effect is bismuth subsalicylate, found in many over-the-counter stomach remedies. Iron supplements, commonly taken for anemia, are also a frequent and benign cause of black or dark green stool. Consuming large quantities of certain dark-colored foods can temporarily change the stool’s color, including:
- Black licorice
- Blueberries
- Dark chocolate
- Blood sausage
Importantly, discoloration from these non-bleeding sources is generally not sticky or tarry, and it lacks the characteristic putrid odor associated with true melena.
Immediate Steps and Medical Evaluation
If black diarrhea is accompanied by symptoms of significant blood loss, such as dizziness, weakness, or a rapid heart rate, emergency medical attention is necessary. These symptoms suggest hemodynamic instability and require immediate resuscitation. Even without these urgent symptoms, any black, tarry stool should prompt an urgent evaluation by a healthcare provider. The evaluation begins with a thorough medical history and diagnostic procedures, including blood tests to check for anemia. The primary diagnostic tool is an upper endoscopy, which uses a flexible tube with a camera to visualize the esophagus, stomach, and duodenum, allowing the doctor to locate and often treat the bleeding source directly.