With proper screening, colon cancer should be detected BEFORE the development of symptoms, when it is most curable.
Most cases of colon cancer have no symptoms. The following symptoms, however, may indicate colon cancer:
Diarrhea, constipation, or other change in bowel habits that does not resolve
Blood in the stool
Unexplained anemia (anemia in any adults other than menstruating women should almost always be evaluated by a colonoscopy)
Abdominal pain and tenderness in the lower abdomen
Weight loss with no known reason
Stools narrower than usual