Colon cancer affects the stool by causing alternating diarrhea and constipation, according to the American Cancer Society. This type of cancer can also cause narrowing of the stool and blood in the stool.
Blood might not be seen in the stool, claims the American Cancer Society, or might make the stool darker. The problem with the above symptoms is that they can also be the symptoms of other conditions such as irritable bowel syndrome, Crohn's disease or hemorrhoids. The consistency of the patient's stool can also change, according to the Mayo Clinic. The patient may also have the feeling that she has not evacuated her bowels completely. If these symptoms persist, the patient should see her doctor to make sure that no malignancy is present.
Another problem with colon cancer is that in many cases the symptoms are silent in the early stages of the disease, says the Mayo Clinic. This is why it's a good idea for a person to get regular screenings for colon cancer when she turns 50. The schedule of screenings might increase if the patient is at high risk for the disease. Risks include being African American, having an inflammatory bowel disease such as Crohn's disease and having a history of intestinal polyps.