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What is colonoscopy?

Colonoscopy is a procedure a doctor uses to look at the inside of the colon and rectum with a colonoscope, which is a long, flexible tube about the width of a finger with a light and 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.

(Note: This test is different from a virtual colonoscopy (also known as CT colonography), which is a type of CT scan.)

Why do you need a colonoscopy?

There are a few reasons you might need a colonoscopy:

To detect (check for) polyps or cancer in the colon and rectum

This test can be used to screen for colorectal cancer. Screening is looking for cancer in people who don’t have symptoms. If abnormal areas are seen during the colonoscopy, they can be removed (biopsied) and tested for cancer. This is done by passing long, thin instruments down the colonoscope, such as small forceps (tweezers) to collect the samples. A screening colonoscopy can also prevent some colorectal cancers, by finding and removing polyps (growths on the inner lining) before they turn into cancer.

If you have had colon or rectal cancer, colonoscopy can also be used to detect new tumors.

Because you are having problems in the colon or rectum

Colonoscopy can also be used to look for the causes of signs or symptoms that might be from colon or rectal cancer (such as changes in bowel movements, bleeding from the rectum, or unexplained weight loss).

You have a problem in the colon or rectum that needs treatment

Colonoscopy can be used to treat some problems in the colon or rectum. For example, instruments can be passed down a colonoscope to remove polyps or other small tumors (growths) in the colon or rectum.

For some advanced cancers that can’t be removed (because they are too big or have spread), a colonoscope can be used to place a rigid tube called a stent into part of the colon or rectum to help keep it open.

What’s it like to have a colonoscopy?

This is a general outline of what typically happens before, during, and after a colonoscopy. But your experience might be a little different, depending on why you’re having it, where you’re having it done, and your overall health. Be sure to talk to your health care provider before having this test so you understand what to expect, and ask questions if there’s anything you’re not sure about.

Before the test

Be sure your health care provider knows about any medicines you are taking, including vitamins, herbs, and supplements, as well as if you have allergies to any medicines (including any type of anesthesia).

You may be asked to stop taking blood-thinning medicines (including aspirin) or some other medicines for several days before the test.

The colon and rectum must be empty and clean so your doctor can see the entire inner lining during the test. You might hear this referred to as a “bowel prep.” There are different ways to do this, including pills, fluids, and enemas (or combinations of these). For example, you might need to drink large amounts of a liquid laxative solution the evening before the procedure. This can often result in spending a lot of time in the bathroom. Because the process of cleaning out the colon and rectum can be unpleasant, it might keep some people from getting this test done. However, newer kits are available to clean out the bowel that might make it easier. Talk to your health care provider about all of your options for the bowel prep.

Your health care provider will give you specific instructions. It’s important to read them carefully a few days ahead of time, since you may need to shop for supplies and laxatives and follow a special liquid diet for at least a day before the test. If you’re not sure about any of the instructions, call the health care provider’s office and get your questions answered.

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

Because a sedative is used to help keep you more comfortable during the test, you will most likely need to arrange for a ride home after the test. You might need someone to help you get home if you are sleepy or dizzy, so many centers that do colonoscopies will not discharge people to go home in a cab or a ridesharing service. If transportation might be a problem, talk with your health care provider about the policy at your hospital or surgery center for using one of these services. There may be other resources available for getting home, depending on the situation.

Getting the test

Colonoscopy can usually be done as an outpatient procedure (where you don’t need to stay overnight in a hospital).

Before the test starts, you’ll likely be given a sedative (into a vein, or IV) to make you feel relaxed and sleepy during the procedure. For most people, this medicine makes them unable to remember the procedure afterward.

During the test, you’ll be asked to lie on your side with your knees pulled up. Your doctor might insert a gloved finger into the rectum to examine it before putting in the colonoscope. The colonoscope is lubricated with gel so it can be inserted easily into the rectum. It is then passed all the way up to the beginning of the colon, called the cecum. If you’re awake, you might 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 and to perform the test. To ease any discomfort, it may help to breathe deeply and slowly through your mouth. The doctor will look carefully at the inner walls of the colon and rectum as the colonoscope is slowly removed.

If a small polyp is found, it may be removed and then sent to a lab to check if it has any areas that have cancer. This is because some small polyps may become cancer over time. If your doctor sees a larger polyp or tumor, or anything else abnormal, a small piece of it will be removed (biopsied) through the colonoscope. It will be checked in the lab to see if it’s cancer, a benign (non-cancerous) growth, or something else.

The test itself usually takes about 30 minutes, but it may take longer if one or more polyps is found and removed.

After the test

After the procedure, you will be watched closely for a while to make sure you don’t have any complications. You might need to stay at the center for about an hour before you are able to go home, and will need a ride home because of the medicines or anesthesia you received. Your doctor or nurse should give you specific instructions on what you can and can’t do in the hours after the test.

Because air is pumped into the colon and rectum during the test, you might feel bloated, have gas pains, or have cramping for a while after the test until the air passes out.

If biopsies were done as part of the procedure, the results will typically be available within a few days, although some tests on the biopsy samples might take longer. You will need to follow up with your doctor after the procedure to get your results.

Possible complications of colonoscopy

Colonoscopy is usually safe, but there is a risk of:

  • Bleeding. If a polyp is removed or a biopsy is done during the colonoscopy, you might notice some blood in your stool for a day or two after the test. Serious bleeding is uncommon, but in rare cases, bleeding might need to be treated or can even be life-threatening.
  • Perforation (puncture the wall of the colon or rectum). This is rare, but it can be a life-threatening complication, and the hole may need to be repaired with surgery.
  • Reactions to anesthesia

Your doctor or nurse should give you specific instructions on what types of problems might require you to call the doctor’s office or seek medical help right away. Be sure you understand these instructions.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

American College of Surgeons. Colonoscopy. 2017. Accessed at on November 13, 2018.

American College of Gastroenterology. Colonoscopy. 2018. Accessed at on November 13, 2018.

National Institute of Diabetes and Digestive and Kidney Diseases. Colonoscopy. 2017. Accessed at on November 13, 2018.

UpToDate. Colonoscopy (Beyond the Basics). 2018. Accessed at on November 14, 2018.

Last Revised: October 3, 2023

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