Why has my stool been black for the past two days?

Black stool, also known as melena, can be caused by various factors. The following are some common causes:

1. Dietary factors: Consuming foods high in melanin, such as black sesame seeds, dark chocolate, and animal blood products like pig blood or duck blood, may darken the stool. Additionally, eating large amounts of iron-rich foods, such as animal offal, may cause the iron to combine with sulfides and form iron sulfide, resulting in black stool.

2. Medications: Taking certain medications, such as iron supplements, bismuth preparations, or activated charcoal, can cause a change in stool color. These medications react with sulfides in the intestines, causing the stool to appear black.

3. Gastrointestinal bleeding: Blood remaining in the gastrointestinal tract for a prolonged period may turn black under the action of gastric acid and intestinal fluids. The iron in hemoglobin is oxidized into iron sulfide, resulting in black stool. The bleeding site may be located in the upper gastrointestinal tract, such as the stomach or duodenum, or in the lower gastrointestinal tract, such as the terminal small intestine or colon, although bleeding from the upper gastrointestinal tract is more common.