What causes black stools after eating watermelon?

Eating watermelon and having black stools may be caused by factors such as diet, medication, or underlying medical conditions.

1. Dietary factors

Under normal circumstances, eating watermelon does not lead to black stools. However, this phenomenon may occur if the person consumes large amounts of green vegetables or blood-based products at the same time as eating watermelon. These foods often contain iron, which can result in black stools after eating watermelon.

2. Medication factors

Black stools may also occur if the person is taking iron supplements while eating watermelon. This is mainly because iron medications react with hydrogen sulfide in the intestines to form black ferrous sulfide, leading to dark-colored stools.

3. Medical conditions

If a patient already has gastrointestinal diseases such as gastric ulcers, consuming watermelon—being a cold-natured fruit—may irritate the gastrointestinal tract, worsen the condition, or even cause gastrointestinal bleeding. Blood that enters the digestive tract remains there for some time and becomes oxidized before being excreted with stool, resulting in the appearance of black stools after eating watermelon.