A colonoscopy is a procedure in which a flexible camera (colonoscope) is passed through the rectum and along the entire large bowel to look for abnormalities including polyps, inflammation, bleeding sources and cancer. It is the gold-standard test for colorectal cancer screening and can be therapeutic — polyps can be removed during the same procedure.
Why colonoscopy matters for cancer prevention
Colorectal cancer is among the most preventable cancers because it typically develops from benign polyps over 10–15 years. Removing polyps during colonoscopy eliminates the cancer before it develops. Studies show that screening colonoscopy reduces colorectal cancer mortality by 50–70% in screened populations.
Most guidelines recommend screening beginning at age 45–50 in average-risk adults, and earlier for those with a family history of colorectal cancer or polyps, or symptoms such as rectal bleeding or changes in bowel habits.
The preparation: the hardest part
A clean bowel is essential for an accurate colonoscopy — any residue can hide polyps. Preparation involves a clear-liquid diet the day before and a bowel-cleansing laxative solution (polyethylene glycol or sodium picosulphate) taken in divided doses the evening before and the morning of the procedure.
The preparation causes several hours of loose, watery stools. Plan to stay near a bathroom. Staying well-hydrated with clear fluids throughout prevents dehydration. Some people find the taste of the prep solution unpleasant — many can be chilled, mixed with clear juice or taken through a straw to make it more tolerable.
What happens during the procedure
Most colonoscopies are performed with conscious sedation — you'll be drowsy and comfortable but not fully asleep. The procedure itself takes 20–45 minutes. You'll lie on your left side as the colonoscope is gently guided around the colon. Air or CO₂ is used to inflate the bowel for better visibility, which can cause mild cramping.
If polyps are found, they are typically removed immediately using a snare or forceps passed through the colonoscope. You will not feel this. Biopsies of unusual tissue are taken for laboratory analysis.
After the procedure
You will need someone to drive you home — sedation impairs judgement and reaction time for several hours. Bloating and wind from the gas used during the procedure is common and resolves within a few hours. Mild cramping is normal; significant abdominal pain, bleeding, or fever warrants a call to your doctor.
If polyps were removed, you may see a small amount of blood in the stool for 1–2 days. A significant amount of rectal bleeding or blood that doesn't stop requires urgent attention.
Understanding the results
- No polyps found, normal bowel — repeat screening in 10 years for average-risk individuals
- 1–2 small tubular adenomas (< 10 mm) — repeat in 3–5 years
- 3–4 small adenomas, or any adenoma ≥ 10 mm — repeat in 3 years
- 5 or more adenomas, or high-grade dysplasia — repeat in 1 year
- Cancer detected — referral for surgical oncology assessment
"Bowel prep is unpleasant. Bowel cancer treatment is worse. The case for colonoscopy is not subtle."
Sara Nassereddin, Health Technology Writer, Looms