What Is a Colonoscopy? Everything You Need to Know

Updated: Oct. 10, 2022

Regular screening with colonoscopy exams can prevent colon cancer or catch it before it spreads. Knowing exactly what to expect before, during, and after your procedure can demystify the process.

What to know about colonoscopies

Colonoscopies save lives.

There is no disputing this fact, yet many people avoid this gold-standard screening test for colon cancer—a leading cause of cancer death for both men and women—because of its invasive nature and the less-than-pleasant prep process.

Again, regular screening with colonoscopy exams can prevent colon cancer. The test allows your doctor to examine your colon (large intestine) and remove pre-cancerous polyps before they morph into cancer.

These tests can also identify colon cancer earlier when the odds of beating it are in your favor. Catching colorectal cancer before spreading makes the five-year relative survival rate about 90 percent, but too few colorectal cancers are found at this stage.

“The goal is to identify the polyp and remove it, because if it is allowed to stay there long enough, it will become a colon cancer,” says Julius Wilder, MD, PhD, an assistant professor of medicine in the division of gastroenterology at Duke University School of Medicine in Durham, North Carolina.

It typically takes about 10 years for a small polyp to develop into cancer.

With that in mind, here’s what you need to know about colonoscopies, including how long it takes, how to prepare, and how often to have this procedure.

Who should get a colonoscopy?

The current screening guidelines state that normal-risk individuals should now get their first colonoscopy starting at age 45.

In the past, the starting age was 50. The change in recommendations is due to an uptick in the number of younger people diagnosed with colon cancer.

“You can have colon cancer growing inside of you and not realize it as it is often asymptomatic,” Dr. Wilder says.

Signs of colon cancer may include:

  • Persistent abdominal pain
  • Blood in stool

Still, there are reasons you may need one earlier, says Dr. Wilder.

These include:

The timing of your next screening colonoscopy is based on the results of your last one. “Talk to your primary care provider about when to start, as well as how frequently you need follow-up exams,” Dr. Wilder says.

(Here’s what you need to know about rectal cancer.)

How to prepare for a colonoscopy

Many people fear the colonoscopy prep. While it’s not fun, it’s really not that bad, says Alia Hasham, MD, a gastroenterologist at Montefiore Medical Center and an assistant professor at Albert Einstein College of Medicine in New York City.

Your doctor needs to get a clear view of the entire inner lining of your colon during the test, and bowel prep is the only way to make sure this happens, she says.

“The preps are safe and well-tolerated, and we will pick the prep that is safest for that patient,” she says.

Types of bowel preps

Bowel preps may involve pills, fluids, and enemas (a fluid injection), or combinations of all of these.

You may need to drink large amounts of a liquid laxative the evening before your procedure. Some common ones include polyethylene glycol (PEG) derivatives such as HalfLytely, GoLytely, and Colyte. These cause watery diarrhea.

Other preps such as OsmoPrep and Visicol are sodium phosphate derivatives. These make your bowel movements more frequent, loose, and runny.

The National Kidney Foundation warns that people with chronic kidney disease (CKD) should avoid sodium phosphate preps, as they can increase the risk of sudden loss of kidney function, leading to kidney failure.

(These are the best colonoscopy prep tips, according to doctors.)

What to eat before a colonoscopy

You will also receive a list of what you can, and can’t, consume during the prep period. Following these instructions closely will increase the chances that your prep is successful.

For example, you may need to stop taking blood-thinning medicines, including aspirin, for several days before the test, as these drugs can increase the risk for bleeding.

You should start eating a low-fiber diet in the days leading up to your colonoscopy, too. The reason? Fibrous foods fill up your colon and take several days to clear.

You may be told that you can only consume clear liquids like clear broth, juice, soft drinks, or sports drinks during the prep process. Eating or drinking anything red, purple, or orange can make it harder for your doctor to see what is happening in your colon. You will also be told not to eat or drink anything after a certain time on the night before your test.

Different doctors prefer different methods on how best to empty your colon. Make sure you are clear about all of the instructions.

anonymous gastroenterologist pointing to a model colon in a doctors officeLiudmila Chernetska/Getty Images

What is a colonoscopy?

The test itself takes about 45 minutes. It may take longer if a polyp requires removal, Dr. Hasham says.

You may receive some type of anesthesia or sedative before the colonoscopy to be more comfortable during the procedure. You may be awake, but not fully aware during the test.

Next, you will lie on your side with your knees pulled up. Your doctor may insert a gloved finger into your rectum to examine it before the test.

For the test, your doctor inserts a flexible tube with a camera on the end (colonoscope) to see the inside of your colon on a video monitor.

It’s put in through your anus and into your rectum and colon. The colonoscope is lubricated. Air may be injected into your bowel to expand it for a better view. Your doctor may also use a water jet to clean the lining of your colon to see it more clearly.

Special instruments can be passed through the colonoscope to remove a small sample of tissue or a polyp for further analysis.

Your doctor may be able to tell if a polyp is pre-cancerous or cancerous just by looking at it, but the sample must go to a lab for a biopsy to be sure, Dr. Wilder says.

The results of this analysis will guide the next steps. It typically takes seven to 10 days to receive the results, he says.

Not all polyps are safely removable during a screening colonoscopy, adds Dr. Hasham. “It is really based on the size and the location of the polyp and how deep it extends.”

If doctors can’t safely remove it during the colonoscopy, you will likely need to see a colorectal surgeon.

Colonoscopy recovery

You may be drowsy from the anesthesia or bloated and gassy from bowel prep and/or any air pumped into your colon during the procedure. This usually resolves within 12 to 24 hours.

“Take it easy on the day of [the] test,” Dr. Hasham says. “You should not go to work or drive.” You will need someone to drive you home after your procedure.

If a polyp is removed, you may experience some bleeding following your colonoscopy.

“The colonoscopy is a very safe procedure with very low rates of any adverse outcomes,” says Dr. Wilder.

Risks may include:

  • Bleeding
  • Perforation of the colon
  • A bad reaction to the sedative
  • Severe belly pain

Bleeding and perforation are the most common complications from colonoscopy, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

Bleeding is more common if you have polyps removed. Delayed bleeding may occur for up to two weeks after the procedure.

Call your doctor if you experience:

  • Persistent or worsening pain or bloating in your belly
  • Blood in stool
  • Fever or chills

What to eat after a colonoscopy

What you eat after the procedure also matters.

Ask your doctor for specific guidance, but in general, it’s best to eat soft, easily digestible foods that are gentle on your digestive system and won’t irritate your colon.

A soft, low-residue diet calls for avoiding any food made with seeds, nuts, whole-grain products, breads and cereals, raw fruits, or vegetables, and limiting dairy products and high-fat foods.

You can eat:

  • Saltines
  • Graham crackers
  • Soup
  • Applesauce
  • Scrambled eggs
  • Jell-O
  • Mashed or baked potato
  • White bread or toast
  • Rice

It’s important to drink lots of fluid, too.

Screening for colon cancer: Other options

Colonoscopy is the gold standard for colorectal cancer screening, but other, non-invasive stool tests are available.

The main advantage compared with colonoscopy is that there is no prep work whatsoever.

“The main disadvantage is that they don’t allow for an examination of your colon,” Dr. Wilder says. “If they are negative, it’s reassuring, but if it is positive, you will need a diagnostic colonoscopy.”

Some people can’t undergo a colonoscopy including:

  • Pregnant women
  • People aged 75 years or older
  • Anyone with limited life expectancy
  • People with severe medical problems

There are several types of stool-based tests available, according to the American Cancer Society.

The fecal immunochemical test (FIT) looks for hidden blood in the stool, which may be an early sign of colon cancer. It can detect blood that comes from your lower intestines, not further up your digestive tract, according to the Colorectal Cancer Alliance.

Fecal occult blood test (FOBT) looks for hidden blood in your stool. It can’t determine if the blood is from your colon, rectum, or other parts of your digestive tract.

Stool DNA testing is a newer type of non-invasive stool-based test to check for colorectal cancer. It looks for certain DNA or genetic changes that may suggest pre-cancerous growths or cancer cells, explains the Colorectal Cancer Alliance.

The last word

Don’t put off your regular screening colonoscopy. “This is a screening test that does a fantastic job of eliminating your risk for the disease of concern,” Dr. Wilder says.

Next, check out the everyday habits that prevent colon cancer.