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Did you know that colorectal cancer is the only cancer that can actually be prevented with a screening exam? Through screening tests, like colonoscopy, cancer-causing polyps can be found and removed early - before they become cancerous. 

Getting regular screening tests for colon cancer is the single best way to protect yourself from the disease. It can catch cancer early, when it's most treatable, and help prevent the disease by finding abnormal growths called polyps that can turn into cancer.

During a colonoscopy, the doctor looks at the entire length of the colon and rectum with a colonoscope, a thin flexible lighted tube with a small video camera on the end. It’s put in through the anus and into the rectum and colon. Special instruments can be passed through the colonoscope to biopsy (sample) or remove any suspicious-looking areas such as polyps, if needed.

Before the test: Be sure your doctor knows about any medicines you are taking. You might need to change how you take them before the test. The colon and rectum must be empty and clean so your doctor can see their inner linings during the test. This can be done many ways, but the most common involves drinking large amounts of a liquid laxative the evening before and the morning of the procedure. This leads to spending a lot of time in the bathroom. 

Your doctor will give you specific instructions. It’s important to read these carefully a few days ahead of time, since you may need to follow a special diet for at least a day before the test and to shop for supplies and laxatives. If you’re not sure about any of the instructions, call the doctor’s office and go over them with the nurse. 

You will probably also be told not to eat or drink anything after midnight the night before your test. If you normally take prescription medicines in the mornings, talk with your doctor or nurse about how to manage them for the day. 

Because a sedative is used during the test, you’ll need to arrange for someone you know to take you home from the test.

During the test: The test itself usually takes about 30 minutes, but it may take longer if a polyp is found and removed. Before it starts, you’ll be given a sedating medicine (into a vein) to make you feel relaxed and sleepy during the procedure. For most people, this medicine makes them unaware of what’s going on and unable to remember the procedure afterward. You’ll wake up after the test is over, but might not be fully awake until later in the day. 

During the test, you’ll be asked to lie on your side with a drape covering you. Your blood pressure, heart rate, and breathing rate will be monitored during and after the test. 

Your doctor might insert a gloved finger into the rectum to examine it before putting in the colonoscope. The colonoscope is lubricated so it can be inserted easily into the rectum. Once in the rectum, the colonoscope is passed all the way to the beginning of the colon, called the cecum. 

If you’re awake, you may feel an urge to have a bowel movement when the colonoscope is inserted or pushed further up the colon. The doctor also puts air into the colon through the colonoscope to make it easier to see the lining of the colon and use the instruments to perform the test. To ease any discomfort, it may help to breathe deeply and slowly through your mouth.

The doctor will look at the inner walls of the colon as he or she slowly removes the colonoscope. If a small polyp is found, it may be removed. This is because some small polyps may become cancer over time. Removing the polyp is usually done by passing a wire loop through the colonoscope to cut the polyp from the wall of the colon with an electric current. The polyp is then sent to a lab to be checked to see if it has any areas that have changed into cancer. 

If your doctor sees a larger polyp or tumor or anything else abnormal, a biopsy may be done. A small piece of tissue is taken out through the colonoscope. The tissue is checked in the lab to see if it’s cancer, a benign (non-cancerous) growth, or a result of inflammation. 

You can read more about colonoscopy and sigmoidoscopy from the American Cancer Society in Frequently Asked Questions About Colonoscopy and Sigmoidoscopy.