Colorectal cancer is cancer that starts in the colon or rectum. It is most commonly caused by abnormal cells that accumulate in the lining of the colon that form polyps. Over time, these polyps become cancerous. Some of the symptoms of colorectal cancer include constipation, diarrhea, blood in the stool, bleeding, excessive gas, unexplained weight loss and abdominal cramps and pain. Men and women over the age of 50 are at the highest risk for colon cancer. You also can have a increased risk if you have history of colon polyps or bowel disease, are overweight, drink heavily, have type 2 diabetes or have a family history of colorectal cancer.