Some of the main causes of blood in the stool are diverticular disease, anal fissures, colitis and angiodysplasia, according to WebMD. Other causes of blood in the stool are peptic ulcers, polyps, cancer or health issues related to the esophagus.
Small pouches in the colon wall called diverticula may bleed or become infected, which can lead to bloody stools, notes WebMD. Passing a large, hard stool can cause the lining of the anus to become cut or torn, which can cause blood in the stool. If the colon becomes inflamed, there may be blood in the stool. An inflamed colon is usually caused by inflammatory bowel disease or an infection.
Abnormal fragile blood vessels may cause blood in the stool, states WebMD. Taking anti-inflammatory drugs, such as naproxen, aspirin or ibuprofen, can cause peptic ulcers, which may lead to bloody stools. Bacterial infection can also cause peptic ulcers.
Colorectal cancer can cause blood in the stool, but the blood may not be visible with an unassisted human eye, according to WebMD. Benign growths called polyps may eventually grow, bleed and possibly become cancerous. A tear in the esophagus or varicose veins of the esophagus can cause blood loss, which can appear in stools.