What Is a Colonoscopy?

A colonoscopy is a medical procedure that allows your doctor to examine the entire inside of your colon (large intestine) and rectum using a thin, flexible tube with a camera attached called a colonoscope. Colonoscopy is considered the gold standard for detecting colorectal cancer and precancerous polyps, which are small growths that can develop into cancer over time.

During the procedure, your doctor can not only look for problems but also remove polyps or take tissue samples for testing—all in the same session. This ability to both diagnose and treat during one examination is a major advantage of colonoscopy.

Why Would I Need a Colonoscopy?

There are several reasons your doctor may recommend a colonoscopy.

Screening for Colorectal Cancer

Colorectal cancer is the third most common cause of cancer death in both men and women in the United States. Screening with colonoscopy can prevent cancer by finding and removing polyps before they turn into cancer and can also detect cancer at an early, more treatable stage.

You should begin screening at age 45–50 if you are at average risk and continue until age 75. Your doctor may recommend starting earlier if you have:

• A family history of colorectal cancer or polyps
• Inflammatory bowel disease (Crohn’s disease or ulcerative colitis)
• Genetic syndromes such as Lynch syndrome or familial adenomatous polyposis
• Previous radiation therapy to the abdomen or pelvis

Follow-Up of Abnormal Tests

If you’ve had an abnormal result on another screening test (such as a stool test for blood), colonoscopy is used to investigate further.

Evaluation of Symptoms

Your doctor may recommend a colonoscopy if you have symptoms such as:

• Rectal bleeding
• Changes in bowel habits
• Abdominal pain
• Unexplained weight loss
• Anemia (low blood count)

Surveillance After Polyp Removal

If you’ve had polyps removed in the past, you may need periodic colonoscopies to check for new polyps.

What Are the Benefits of Colonoscopy?

Prevents Cancer and Saves Lives

Research shows that colonoscopy reduces the risk of colorectal cancer by approximately 69% and reduces the risk of dying from colorectal cancer by 68%. The procedure is particularly effective at preventing cancer in the lower part of the colon (about 80%) and provides 40–60% protection in the upper colon.

In one large study, people who underwent screening colonoscopy had a 67% lower risk of dying from colorectal cancer, including a 65% reduction in deaths from cancer in the upper colon.

High Accuracy

Colonoscopy has approximately 95% sensitivity for detecting cancer and precancerous polyps, making it more accurate than other screening options.

One-Stop Diagnosis and Treatment

Unlike tests that only detect abnormalities, colonoscopy allows your doctor to remove polyps during the same procedure, eliminating the need for a second appointment.

Long Intervals Between Screenings

If your colonoscopy is normal, you typically won’t need another one for 10 years. One or two negative examinations may provide long-term or even lifetime protection against colorectal cancer.

Examines the Entire Colon

Unlike sigmoidoscopy, which examines only the lower colon, colonoscopy evaluates the entire large intestine.

How Do I Prepare for a Colonoscopy?

Proper preparation is essential for an accurate and effective colonoscopy. The quality of bowel preparation directly affects how well polyps and other abnormalities can be detected.

Before the Procedure

• Arrange transportation: Sedation is used, so someone must drive you home
• Plan time off: Take the day of the procedure off from work or activities
• Medication adjustments: Tell your doctor about all medications, especially:
  • Blood thinners (anticoagulants or antiplatelet drugs)
  • Diabetes medications
  • Blood pressure medications
  • GLP-1 receptor agonists (such as Ozempic or Wegovy)
• Dietary changes: You’ll usually switch to a liquid diet one or more days before the exam

Bowel Preparation (Bowel Prep)

You must completely empty your colon using a special cleansing solution.

• Split-dose prep (most effective):
  • Drink half the solution the evening before the colonoscopy
  • Drink the remaining half the morning of the procedure
• Low-volume prep options may be available and easier to tolerate

It is critical that you follow instructions exactly, drink the full amount of preparation, complete it at the correct times, and continue clear liquids as directed.

What Happens During the Colonoscopy?

Location and Duration

Colonoscopy is typically performed in an outpatient hospital department or endoscopy center. The procedure itself usually takes 20–45 minutes, though you should plan for 2–3 hours total for check-in, preparation, the exam, and recovery.

During the Procedure

1. Sedation: You’ll receive medication to keep you relaxed and comfortable. Most patients do not remember the procedure.
2. Positioning: You’ll lie on your side on an exam table.
3. The examination:
   • The colonoscope is gently inserted into the rectum and advanced through the colon
   • Air or carbon dioxide is used to expand the colon for better visibility
   • The lining of the colon is carefully examined during insertion and withdrawal
   • The exam ends when the scope reaches the cecum (near the appendix)
4. Polyp removal: Polyps or abnormal tissue can be removed or biopsied during the same procedure and sent for laboratory analysis.

After the Procedure

• You’ll recover for 30–60 minutes while sedation wears off
• Mild bloating or cramping is common and temporary
• You must have someone drive you home
• Most patients can eat normally unless instructed otherwise