Colorectal cancer may cause one or more of the symptoms below. If you have any of the following you should see your doctor:

• A change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts for more than a few days
• A feeling that you need to have a bowel movement that is not relieved by doing so
• Rectal bleeding
• Blood in the stool which may make it look dark
• Cramping or abdominal (belly) pain
• Weakness and fatigue
• Unintended weight loss

Colorectal cancers can bleed. While sometimes the blood can be seen or cause the stool to become darker, often the stool looks normal. The blood loss can build up over time, though, and lead to low red blood cell counts (anemia). Sometimes the first sign of colorectal cancer is a blood test showing a low red blood cell count.

Most of these problems are more often caused by conditions other than colorectal cancer, such as infection, hemorrhoids, irritable bowel syndrome, or inflammatory bowel disease. Still, if you have any of these problems, it’s important to see your doctor right away so the cause can be found and treated, if needed.

Regular screening can often find colorectal cancer early, when it is most likely to be curable. In many people, screening can also prevent colorectal cancer altogether. This is because some polyps, or growths, can be found and removed before they have the chance to turn into cancer.

Tests used to screen for colorectal cancer include:

• Guaiac-based fecal occult blood test (gFOBT) and fecal immunochemical test (FIT): Samples of stool (feces) are checked for blood, which might be a sign of a polyp or cancer.
• Stool DNA test: A sample of stool is checked for certain abnormal sections of DNA (genetic material) from cancer or polyp cells.
• Sigmoidoscopy: A flexible, lighted tube is put into the rectum and lower colon to check for polyps and cancer.
• Colonoscopy: A longer, flexible tube is used to look at the entire colon and rectum.
• Double contrast barium enema: This is an x-ray test of the colon and rectum.
• CT colonography (virtual colonoscopy): This is a type of CT scan of the colon and rectum.

gFOBT, FIT, and stool DNA testing mainly find cancer, but can find some polyps.

Sigmoidoscopy, colonoscopy, double contrast barium enema, and CT colonography are good at finding cancer and polyps. Polyps found before they become cancer can be removed, so these tests may prevent colorectal cancer. This is why these tests are preferred if they are available and you are willing to have them.