 Hello, I am Dr. Kakoli Lhakar, Consultant Medical Alchemist in Manipal Hospital, Bangalore. The most common symptom of colon cancer is alteration in the bowel habits. It can either be constipation or diarrhea or constipation alternating with diarrhea for more than a few weeks. So, if there is a change in the bowel habits for more than a few weeks, it is always better to get yourself checked. Then the second most common symptom is dark stools or blood in the stools. Also, there can be a feeling that bowel movement is not adequately relieved, that is another very common symptom. Besides these, there can be weakness and fatigue and unexplained weight loss. Also, there is a golden rule that any anemia in a man and in a woman who is post-monopausal should be evaluated for colon cancer.