Colorectal cancer is cancer that begins in the colon or the rectum. It is the third most common cancer among both men and women in the United States, and it occurs most often in people over the age of 50. This fact sheet answers some commonly asked questions about colorectal cancer screening.

Why is screening for colorectal cancer so important?

Many colorectal cancers can be prevented through regular screening. Screening can find precancerous polyps— abnormal growths in the colon or rectum—so that they can be removed before they turn into cancer.

Screening is crucial because when found early, colorectal cancer is highly treatable. Early stages of colorectal cancer usually present no symptoms, which tend to appear as the cancer progresses.

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What is a colonoscopy?

A colonoscopy is the best screening test available for colorectal cancer. It is the only screening test that also prevents many colorectal cancers. Here are a few things you should know about this test:

  • During a colonoscopy, your doctor examines the lining of your entire colon to check for polyps or tumors. If any polyps are found, they can be removed immediately.
  • On the day of the colonoscopy, you will receive medication to help you relax. Most people fall asleep and do not remember much about the test when they wake up.
  • Your doctor performs a colonoscopy by inserting a long, thin, flexible tube called a colonoscope into your colon through the rectum. The tube has a tiny video camera and light at the end that sends images to a video monitor.
  • Before the procedure, you will receive instructions from your doctor on what to eat and how to empty your bowel.
  • Both men and women should have a colonoscopy starting at age 50. People at increased risk of colorectal cancer may start earlier, depending on your doctor’s instructions. (See “Risk Factors” on back). Also, your doctor will tell you in how many years you will need another colonoscopy.

Are other screening tests available?

If you are unable to have a colonoscopy, your doctor can give you information about the following tests and how often they should be performed:

  • Flexible sigmoidoscopy
  • Double contrast barium enema
  • Virtual colonoscopy (CT colonoscopy)
  • Fecal occult blood test
  • Stool DNA tests

You should note that these tests are not as thorough as a colonoscopy. If polyps or tumors are suspected based on these tests, you will still need to have a colonoscopy.