Black Stool

Black stool should be always considered as a warning sign that should not be underestimated. Patients with newly-emerged black and smelling stool should promptly visit a doctor.



Melena is the most important cause of black colored stools. It is caused by bleeding into the upper digestive tract (including swallowed blood from the mouth or respiratory system). Blood coming through digestive tract is digested – such blood in stool has tarry appearance, black color and characteristic displeasing odor.

Iron supplements

Patients taking drugs containing (usually people with iron-deficiency anemia) have black- colored stool. It is not tar black like in melena, it has more likely a black-green shade. Iron totally blackens intestinal mucosa; therefore it is practically impossible to perform a high-quality colonoscopy by an iron supplement user.

Food influence

Different types of foods may discolor stools dark or even black. These include red beets, blueberries, etc.

Diagnostic approach

Less serious causes of black stool can be confirmed by cautious medical history. It is important to know patient’s present drug list (iron supplements) and diet adopted in time period before black stool occurrence. In addition, it is important to perform at least a basic physical examination including digital rectal exam. To rule out upper digestive bleeding, esophagogastroduodenoscopy can be performed.


In case of bleeding into the digestive tract, therapy depends on the amount of blood loss and the source of bleeding. If there is severe blood loss with risk of a shock condition, the patient should be hospitalized, stabilized (infusions, oxygen, certain medication, etc.) and only then bleeding shall be located and solved.

Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Sources: basic text sources