After eating half a watermelon, my stool turned red.

Eating half a watermelon and subsequently passing red-colored stool may represent a normal physiological phenomenon, or it could be caused by gastrointestinal bleeding, hemorrhoids, or other factors—treatment should be directed at the underlying cause.

1. Physiological Phenomenon

Red-colored stool after eating half a watermelon may result from plant pigments (e.g., lycopene) in the watermelon that are incompletely absorbed and excreted unchanged in the stool. This is generally a benign, physiological occurrence and does not typically warrant concern.

2. Gastrointestinal Bleeding

If consuming watermelon triggers diarrhea—and if the patient has a prior history of inflammatory bowel disease, peptic ulcer disease, or other conditions associated with gastrointestinal bleeding—this may precipitate a recurrence of bleeding, manifesting as red-colored stool. Under medical guidance, acid-suppressing medications may be prescribed to reduce bleeding; commonly used agents include rabeprazole and omeprazole.

3. Hemorrhoids

In patients with pre-existing hemorrhoids, excessive watermelon consumption leading to diarrhea may increase bowel movement frequency and strain during defecation, potentially causing hemorrhoidal rupture and bleeding—resulting in red-colored stool. Symptom relief may be achieved under physician supervision using traditional Chinese patent medicines, such as Zhisuining Tablets or Huaijiao Wan.