Approved by the Cancer.Net Editorial Board, 03/2020

A sigmoidoscopy is a test that looks at the rectum and lower part of the large intestine. "Colon" is the medical term for the large intestine, and the sigmoid colon is the lower part. The sigmoid colon ends in the rectum. Waste collects in the rectum and leaves your body when you have a bowel movement.

The drawing below shows the different parts of the colon and rectum.

This illustration shows the 5 sections of the colon and rectum. The ascending colon is the beginning the large intestine into which the small intestine empties; it begins on the lower right side of the abdomen and then leads up to the transverse colon. The transverse colon crosses the top of the abdomen from right to left, leading to the descending colon, which takes waste down the left side. Finally, the sigmoid colon at the bottom takes waste a few more inches, down to the rectum. A cross-section of the rectum and sigmoid colon shows where waste leaves the body, through the anus. Copyright 2004 American Society of Clinical Oncology. Robert Morreale/Visual Explanations, LLC.

A sigmoidoscopy is one way to look for colorectal cancer or other growths. This includes small growths called polyps. Polyps are not cancer, but they can turn into cancer. You might also have this test to look for the cause of bowel problems. These can include bleeding, a change in bowel movements, or other problems.

How does a sigmoidoscopy work?

This test uses a thin, flexible tube with a light and camera on one end. Your doctor places this end in the anus. Then they push it gently up through the rectum into the lower colon. They can see inside the colon through a small scope on the tube or on a video monitor.

How is a sigmoidosocopy different from a colonoscopy?

Both a sigmoidoscopy and a colonoscopy are screening tests to look for colorectal cancer. The difference between the two tests is the part of the colon they allow the doctor to see. A sigmoidoscopy is less invasive, because it only looks at the lower part of your colon. A colonoscopy looks at the entire large intestine.

If your doctor finds polyps during your sigmoidoscopy, you will probably need a colonoscopy. It is important because if you have polyps in the lower colon, there is a good chance you have them elsewhere, too. Your doctor can remove the polyps before they turn into cancer.

Who will do my sigmoidoscopy?

A doctor specializing in the digestive system usually does it. It may also be done by a surgeon, family practice doctor, physician assistant, or nurse practitioner. A nurse will probably help with the test.

How should I get ready for a sigmoidoscopy?

When you schedule the test, your doctor's office will tell you how to get ready. Be sure to follow their instructions carefully to avoid affecting your test results. Talk with the staff about the following topics, and ask questions about any information that is unclear or concerning to you.

What to eat. You may need to eat a liquid diet for up to 24 hours before the test. Allowed liquids often include fat-free bouillon or broth, black coffee, strained fruit juice, and gelatin.

Empty your colon. Your lower colon must be empty for the doctor to examine it. You will need an enema or strong laxatives before the test. Be sure to follow your doctor's instructions. You will take this the night before or the same day.

If your doctor prescribes a laxative, it will be a pill or a powder you mix with water before drinking. Once you take it, you will have bowel movements often until your colon is empty. Make sure you are near a bathroom during this time.

What to wear. You will be given a hospital gown to wear during the test. You do not need to wear anything special to your appointment.

Your medications and health history. Ask if you should take your usual medications on the day of the test. This includes any prescription drugs, over the counter drugs, and supplements you take. You may need to stop taking medications that increase your risk of bleeding for a few days before the test. These medications include aspirin and other blood thinners. Also, let the staff know if you have any medical conditions.

Allergies. Let the medical staff know about any drug allergies or other allergies you have.

Insurance, cost, and consent. If you are concerned about the cost of your test, contact your insurance company beforehand. Ask how much of the cost you will have to pay.

When you arrive for your test, you will be asked to sign a consent form. This form says you understand the benefits and risks of the procedure and agree to have it. Be sure to ask the health care team about any questions or concerns you have.

What happens during a sigmoidoscopy?

A sigmoidoscopy usually takes 15 to 20 minutes.

When you arrive for your sigmoidoscopy, you will change into a hospital gown. The doctor will ask you to lie on your side on an exam table. They will put a sheet over your body so everything is covered except the test area.

Then, the doctor will gently place the small, thin tube in your rectum. They will use the tube to blow some air into your colon. The air blows it up a bit so it is easier to see through the scope. You might feel bloated or like you need to go to the bathroom. Next, the doctor will gently push the tube up through your rectum into your lower colon.

If the doctor finds a polyp in your colon, they can remove it or take a sample. They will send it to a laboratory. A specialist called a pathologist will examine the sample under a microscope to look for cancer.

Is a sigmoidoscopy painful?

You may feel some discomfort but not pain. You will probably not have anesthesia, but your doctor may give you medication to help you relax and make you sleepy.

If you do start to feel pain at any time, tell your doctor. They can move the tube to help you feel more comfortable.

What happens after sigmoidoscopy?

You can expect to go back to normal activities immediately. This includes driving, unless the doctor gave you medicine to help you relax. If so, you will be sleepy after the test. You may want to arrange ahead of time for a friend or family member to drive you home.

You might have cramps or feel bloated at first. This usually goes away in a few hours. You might pass gas and even have some diarrhea as you release the air that the doctor put into your colon.

You might notice a small amount of bleeding from your rectum. This is normal. However, call your doctor if you continue bleeding or if you have blood clots. Also call if you have:

  • Abdominal pain (belly pain)

  • Dizziness

  • Weakness

  • Fever of 100 F (37.8 C)

Questions to ask the health care team

Before a sigmoidoscopy, you might want to ask your health care team these questions.

  • Who will do this test? Will anyone else be in the room during the test?

  • What can I eat or drink before the test?

  • Can I take my regular medications the day of the test? If not, when can I take them again?

  • How else can I prepare for this test?

  • What are the benefits and risks of this test?

  • Do I need someone to drive me home afterwards? Should I avoid any activities?

  • When will I get the results, and who will explain them?

  • Will I need other tests?

Related Resources

Types of Endoscopy

Cancer Screening

More Information

National Institute of Diabetes and Digestive and Kidney Diseases: Flexible Sigmoidoscopy

National Cancer Institute: Colorectal Cancer Screening