Local symptoms are more likely if the tumor is located closer to the anus. There may be a change in bowel habit (new-onset constipation or diarrhea in the absence of another cause), and a feeling of incomplete defecation (tenesmus) and reduction in diameter of stool; tenesmus and change in stool shape are both characteristic of rectal cancer. Lower gastrointestinal bleeding, including the passage of bright red blood in the stool, may indicate colorectal cancer, as may the increased presence of mucus. Melena, black stool with a tarry appearance, normally occurs in upper gastrointestinal bleeding (such as from a duodenal ulcer) but is sometimes encountered in colorectal cancer when the disease is located in the beginning of the large bowel. A tumor that is large enough to fill the entire lumen of the bowel may cause bowel obstruction. This situation is characterized by constipation, abdominal pain, abdominal distension and vomiting. This occasionally leads to the obstructed and distended bowel perforating and causing peritonitis. Certain local effects of colorectal cancer occur when the disease has become more advanced. A large tumor is more likely to be noticed on feeling the abdomen, and it may be noticed by a doctor on physical examination. The disease may invade other organs, and may cause blood or air in the urine (invasion of the bladder) or vaginal discharge (invasion of the female reproductive tract).  Constitutional symptoms
If a tumor has caused chronic occult bleeding, iron deficiency anemia may occur; this may be experienced as fatigue, palpitations and noticed as pallor (pale appearance of the skin). Colorectal cancer may also lead to weight loss, generally due to a decreased appetite. More unusual constitutational symptoms are an unexplained fever and one of several paraneoplastic syndrome. The most common paraneoplastic syndrome is thrombosis, usually deep vein thrombosis.  Metastatic symptoms
Colorectal cancer most commonly spreads to the liver. This may go unnoticed, but large deposits in the liver may cause jaundice and abdominal pain (due to stretching of the capsule). If the tumor deposit obstructs the bile duct, the jaundice may be accompanied by other features of biliary obstruction, such as pale stools.  Risk factors
Micrograph of a tubular adenoma, a type of colonic polyp and a precursor of colorectal cancer. The lifetime risk of developing colon cancer in the United States is about 7%. Certain factors increase a person's risk of developing the disease. These include: * Age. The risk of developing colorectal cancer increases with age. Most cases occur in the 60s and 70s, while cases before age 50 are uncommon unless a family history of early colon cancer is present. * Polyps of the colon, particularly adenomatous polyps, are a risk factor for colon cancer. The removal of colon polyps at the time of colonoscopy reduces...