Colonoscopy vs. Sigmoidoscopy

Written by Nicole Schmidt
Medically Reviewed by Melinda Ratini, MS, DO on March 25, 2025
9 min read

If your doctor wants to check the health of your colon — whether as part of a routine screening or to investigate a concern — they may recommend a colonoscopy or sigmoidoscopy. Both tests use a thin, flexible tube with a camera to check the inside of your colon — the part of your digestive system that processes food waste and helps you poop. The main difference is how much of the colon each test examines.

During a colonoscopy, your doctor will look inside your entire large intestine, which is the final part of your digestive system. It’s about five feet long and forms a square-like shape inside your abdomen. The large intestine includes four sections: the sigmoid colon, which connects the colon to the rectum; the descending colon, which runs down the left side of your body; the transverse colon, which stretches across your abdomen; and the ascending colon, which moves upward on the right side.

A sigmoidoscopy, on the other hand, only examines at the lower part of the colon or large intestine. 

Jennifer Davids, MD, chief of colon and rectal surgery at Boston Medical Center, explains that while a colonoscopy provides a more complete evaluation and is often the better diagnostic tool, both tests have pros and cons. “A lot of patients are turned off by stool tests, colonoscopy prep, sedation, or time off from work and may decide not to proceed with any type of screening, which is what we want to avoid,” she says. “The best test is the one that makes you most comfortable. All of the available tests are superior to no test at all.”

Your doctor may recommend a colonoscopy as part of a routine screening to investigate symptoms like pain or bleeding or even as a treatment option. During the procedure, they use a thin, flexible tube with a camera, called an endoscope, to see inside your entire colon. This allows them to check for polyps (abnormal growths), inflammation, or other signs of disease. Unlike MRIs or X-rays, a colonoscopy gives a direct view inside your colon, making it one of the most effective ways to diagnose gastrointestinal conditions. Your doctor may also take a small tissue sample, called a biopsy, for further testing under a microscope. 

If they find a problem during the procedure, they might be able to treat it right away. Using specialized tools passed through the endoscope, they can remove polyps or blockages, seal wounds, inject medication, treat tissue with laser therapy, or place a stent — a small tube that helps keep a narrowed or blocked section of the colon open. In many cases, these treatments can prevent the need for additional procedures or surgery.

Colonoscopy for cancer prevention

A colonoscopy is one of the best ways to prevent colon cancer. Since the disease often doesn’t cause symptoms until it’s more advanced and harder to treat, regular screenings can catch it early—or even stop it from developing in the first place. During the procedure, your doctor looks for polyps — small growths in the colon that can sometimes turn into cancer — and removes them before they become a problem. They can also check for tumors while they’re still in the early, more treatable stages.

In 2021, the recommended age for regular colonoscopy screenings decreased from 50 to 45. “It's very important to get the word out for everybody that's between the ages of 45 and 50 that they need to catch up,” Davids says. If you have a family history of colorectal cancer or conditions like inflammatory bowel disease, your doctor may recommend starting sooner. After you turn 45, you should have a colonoscopy every 10 years, but if you're at higher risk of developing colon cancer, your doctor may suggest more frequent screenings. 

Davids points out that colorectal cancer is also becoming more common in younger people and urges anyone with unusual symptoms to talk to their doctor about screening. “The idea that this is an older person's cancer has really been proven to not be true,” she says. “Anybody at any age should not be afraid to reach out to their physician to get checked if they have concerns.” 

Condition diagnosis and monitoring

A colonoscopy can also be an important tool for diagnosing and monitoring conditions that involve your digestive tract. If you have digestive symptoms without a clear cause — like bleeding, ongoing stomach pain, or changes in your bowel movements — a colonoscopy can help identify issues that cause ongoing inflammation, like ulcerative colitis or Crohn’s disease. It can also detect diverticulosis (a condition that causes small pouches to form inside the colon) and diverticulitis (when those pouches become inflamed or infected). 

A colonoscopy can also help identify: 

  • Ulcers and perforations
  • Bowel obstructions 
  • Intestinal ischemia and ischemic colitis (when parts of your digestive system don’t get enough blood flow)
  • Abnormal growths and colon cancer 

If you’ve already been diagnosed with one of these conditions, your doctor may recommend follow-up colonoscopies to monitor any changes and adjust your treatment as needed. “It's a very valuable way to look at the lining of the bowel to follow diseases and determine treatment,” Davids says. 

A sigmoidoscopy is a procedure that helps doctors examine the lower part of the colon. If your symptoms suggest that there’s an issue in this area, a sigmoidoscopy can be a good option since it's a simpler and less invasive procedure than a full colonoscopy. Your doctor may recommend one if you have any of the following symptoms:

  • Constipation or diarrhea 
  • Belly pain
  • Rectal bleeding 
  • Itching around your anus 
  • Blood or mucus in your stool
  • Low iron or low blood counts 

A sigmoidoscopy can also be used to screen for colon cancer, especially if you’re at lower risk or have health conditions that make a full colonoscopy more challenging. In these cases, doctors typically recommend a sigmoidoscopy every five to 10 years, along with a stool test to check for traces of blood or DNA. Your doctor can tell you how often you'll need these tests. 

Changes in bowel movements

The color, consistency, and frequency of your poop can give you insight into your digestive health. Healthy stool is usually brown, soft and solid, and should pass easily without strain. Bathroom habits vary from person to person, but you should have regular bowel movements — at least once every three days — and control over when you go. Minor changes aren't usually a cause for concern, but larger changes could indicate a problem. These might include blood in your stool, frequent diarrhea or constipation, thin, pencil-like stools, or a constant urge to poop without relief (a condition called tenesmus). 

Davids acknowledges that talking about your bowel habits can feel like a sensitive topic, but any abnormalities that can’t be explained by a change in medication, activity, or diet should be discussed with your doctor. If you have any problems, your doctor may recommend a sigmoidoscopy to check for possible causes.  

Condition diagnosis and monitoring

During a sigmoidoscopy, doctors can check for inflammation and diagnose conditions like polyps, tumors, ulcers, hemorrhoids, and other conditions that affect the lower colon, including: 

  • Ulcerative colitis (UC) and Crohn’s disease. Inflammatory bowel diseases like UC and Crohn’s cause long-term inflammation and ulcers in the digestive tract. This can lead to symptoms like cramps, diarrhea, and other digestive issues. 
  • Diverticulosis and diverticulitis. Diverticulosis happens when small pouches form in the lining of your colon. If these pouches become inflamed or infected, it leads to another condition called diverticulitis. 

If you’ve been diagnosed with one of these conditions, follow-up sigmoidoscopies can help your doctor track changes, evaluate how your treatment is working, and make adjustments as needed.

If your doctor finds polyps, inflammation, or any other problems during the procedure, they may recommend a colonoscopy as part of a follow-up examination.  

During a colonoscopy, a doctor inserts a thin, flexible tube called a colonoscope into your anus and slowly guides it through your large intestine. The tube has a small camera that sends images to a screen, allowing the doctor to see what’s happening inside your colon. To get a clearer view, the doctor will pump air through the tube to gently expand the colon. Once the scope reaches the end of your large intestine, the doctor slowly withdraws it, examining the tissue again as they go. 

In the days leading up to a colonoscopy, you may need to adjust what you eat and drink. The night before, you’ll take a laxative to clear out your colon and you won’t be able to eat again until after the procedure. Your doctor may recommend performing your colonoscopy under general anesthesia, meaning you’ll be fully asleep, or with sedation, which keeps you relaxed and drowsy, but still awake. If you’re awake, you won’t feel pain, but you might experience some pressure or mild discomfort as the colonoscope moves through your intestine. The entire procedure should take between 30 and 45 minutes.

A colonoscopy is usually an outpatient procedure, so you’ll be able to go home the same day. Afterward, you might have some gas, stomach pain, bloating, or nausea. If you had anesthesia, you might also feel drowsy. But these symptoms are normal and should pass within a few hours.

Similar to colonoscopy, a sigmoidoscopy involves a doctor inserting a thin, flexible tube with a small camera attached into your anus. This tube is shorter than the one used for a colonoscopy, since the procedure only examines the lower portion of the intestine. The doctor will advance the tube slowly while inflating your colon with a bit of air to get a clearer picture. This shouldn’t be painful, but it might cause cramps or gas.

The procedure is usually quick, lasting about 15 minutes, though it may take longer if the doctor needs to take a tissue sample or treat a minor issue. Preparing for a sigmoidoscopy is usually easier than for a colonoscopy, but you may need to follow a restricted diet beforehand and use a suppository to empty your bowels. Most people don’t need sedation or anesthesia for this test.

Both colonoscopy and sigmoidoscopy are procedures that help doctors understand what’s happening inside the colon. While a colonoscopy offers a full view of the large intestine and is the best tool for detecting colorectal cancer and other conditions, a sigmoidoscopy is a less invasive option that focuses on the lower colon and can help diagnose localized issues. Regular screenings with either test are crucial for early detection and prevention of colorectal cancer. If you notice changes in your digestion or bowel habits, be sure to talk to your doctor about the best screening option for you.

Why would you have a sigmoidoscopy instead of a colonoscopy? If your doctor suspects an issue in your lower colon, they may recommend a sigmoidoscopy instead of a colonoscopy. Some people also choose sigmoidoscopy for cancer screening because it’s less invasive.

Can colon cancer be detected by sigmoidoscopy? A sigmoidoscopy can detect colon cancer, but only if the cancer is in the lower part of the colon. A full colonoscopy is the best screening method for detecting cancer throughout the entire colon.

Do they put you to sleep for a sigmoidoscopy? A sigmoidoscopy is a quick, typically painless procedure that usually doesn’t require anesthesia or sedation.

Which is more painful: colonoscopy or sigmoidoscopy? Neither procedure is painful, but a colonoscopy requires more preparation and recovery time and you will typically have some type of sedation for a colonoscopy. Both may cause mild discomfort or gas.