In adults, most common causes are hemorrhoids and diverticulosis, both of which are relatively benign; however, it can also be caused by colorectal cancer, which is potentially fatal. In a newborn infant, haematochezia may be the result of swallowed maternal blood at the time of delivery, but can also be an initial symptom of necrotizing enterocolitis, a serious condition affecting premature infants. In babies, haematochezia in conjunction with abdominal pain is associated with intussusception. In adolescents and young adults, inflammatory bowel disease, particularly ulcerative colitis, is a serious cause of haematochezia that must be considered and excluded.
Haematochezia can be due to upper gastrointestinal bleeding. However, as the blood from such a bleed is usually chemically modified by action of acid and enzymes, it presents more commonly as black "tarry" feces known as melena. Haematochezia from an upper gastrointestinal source is an ominous sign, as it suggests a very significant bleed which is more likely to be life-threatening.
Consumption of dragon fruit or pitaya may also cause red discoloration of the stool and sometimes the urine (pseudohematuria). This too, is a differential sign that is sometimes mistaken for hematochezia.
Other common causes of blood in the stool include:
- Colorectal cancer
- Crohns disease
- Ulcerative colitis
- Other types of inflammatory bowel disease, inflammatory bowel syndrome, or ulceration
- Rectal or anal hemorrhoids or anal fissures, particularly if they rupture or are otherwise irritated[來源請求]
- Shigella or shiga toxin producing  E. coli food poisoning
- Necrotizing enterocolitis[來源請求]
- Upper gastrointestinal bleeding
- Peptic ulcer disease
- Esophageal varices
- Gastric cancer
- Intense exercise, especially a high-impact activity like running in hot weather.
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