A colonoscopy is a 20- to 40-minute procedure where a doctor uses a flexible camera to examine the inside of your large intestine. Most people are sedated and feel little to nothing during the exam itself. The part patients find most challenging is almost always the preparation beforehand, not the procedure. Here’s what to expect at each stage.
The Days Before: Dietary Changes
Your doctor will likely ask you to switch to a low-fiber diet about three days before the procedure. This means sticking to foods like white rice, white bread, eggs, tender meat, fish, cheese, yogurt, and well-cooked vegetables like carrots and green beans. You’ll need to avoid nuts, seeds, whole grains, popcorn, dried fruit, raw vegetables, beans, and lentils. The goal is to reduce the amount of residue in your colon so the camera has a clear view.
The day before your colonoscopy, you move to a clear liquid diet from the moment you wake up. That means broth, clear juices (no pulp), popsicles, gelatin, water, and tea or coffee without milk. No solid food at all.
The Prep: What People Dread Most
On the evening before the procedure, typically between 5 and 8 p.m., you’ll drink a large volume of laxative solution. Several formulations exist, but they all do the same thing: empty your colon completely. Some come as a full gallon jug that you drink in large glasses every 10 to 15 minutes over about two hours. Others are smaller-volume solutions followed by extra clear fluids.
The taste is often described as salty or mildly unpleasant. Chilling the solution helps. Within an hour or so of starting, you’ll begin making frequent trips to the bathroom. This continues until your stool runs clear. Most people find this the worst part of the entire experience. Plan to stay close to a bathroom for the rest of the evening, and keep your phone or a book handy. A second dose is usually required the morning of the procedure, several hours before your appointment.
Arrival and Sedation
At the clinic or hospital, you’ll change into a gown, and a nurse will start an IV line. You’ll go over your medical history and sign consent forms. Then you’ll discuss sedation with your care team.
There are two main approaches. The most common is conscious sedation, which uses a combination of a sedative and a pain reliever delivered through your IV. You’ll feel drowsy and relaxed. Some people stay lightly aware during the procedure, while others drift off entirely. The sedative used in conscious sedation often causes amnesia, so even if you were somewhat awake, you probably won’t remember it afterward.
The other option is deep sedation with a faster-acting drug administered by an anesthesiologist or nurse anesthetist. With deep sedation, nearly all patients sleep through the entire procedure and remember nothing. It tends to cost more because of the added anesthesia provider. Your facility may use one approach as their default, or your doctor may give you a choice.
What Happens During the Procedure
You’ll lie on your left side with your knees pulled up toward your chest. The doctor applies a numbing gel to the anal area, then gently inserts the colonoscope, a flexible tube about the width of a finger with a tiny camera and light on the end.
The scope is guided through your rectum, around the bends of your colon, and all the way to where the large intestine meets the small intestine. To get a good view, the doctor inflates the colon slightly with air or carbon dioxide. This is what can cause brief sensations of pressure or cramping if you’re lightly sedated. The doctor navigates around several natural curves in the colon, adjusting the scope’s tip and sometimes asking a nurse to press gently on your abdomen to help the scope advance. Occasionally your position may be shifted onto your back to help the scope pass a tricky bend.
The camera sends a live video feed to a monitor. The doctor examines the entire lining of the colon, looking for polyps, inflammation, or anything unusual. The whole insertion and withdrawal typically takes 20 to 40 minutes. A high-quality exam finds precancerous growths (called adenomas) in at least 25% of screening patients overall.
If Polyps Are Found
Finding polyps is common, and removing them during the same procedure is routine. You won’t feel it happening. For small polyps (up to about 10 millimeters), the doctor threads a thin wire loop called a snare through the scope, lassoes the polyp along with a small margin of surrounding tissue, and clips it off. This “cold snare” technique doesn’t use electrical current and carries very low risk of bleeding. The removed tissue is sent to a lab to determine whether it’s precancerous.
For larger polyps, the doctor may use an electrified snare or other specialized techniques. Occasionally a polyp is too large or positioned in a way that requires a follow-up procedure, but this is uncommon during a routine screening.
Recovery and Going Home
After the scope is removed, you’re wheeled to a recovery area where nurses monitor your vital signs as the sedation wears off. Most people spend roughly an hour in recovery. A nurse will check that you’re alert, oriented, and able to walk steadily before clearing you to leave.
You will need someone to drive you home. Sedation impairs your judgment and reaction time for the rest of the day, even if you feel fine. Plan on not driving, making important decisions, or going back to work that day.
The most common sensation after the procedure is bloating and mild cramping from the air that was pumped into your colon. Walking around and passing gas relieves this, usually within a few hours. You can eat after the procedure, though many people start with something light. If polyps were removed, your doctor may suggest avoiding certain foods or blood-thinning medications for a short period.
How Safe Is It
Colonoscopy is considered very safe. The most serious potential complication is a perforation, a small tear in the colon wall. This happens in roughly 0.03% to 0.2% of diagnostic colonoscopies, meaning it occurs in fewer than 2 out of every 1,000 procedures. The risk is slightly higher when polyps are removed. Bleeding after a polypectomy can also occur but is usually minor and resolves on its own.
Signs of a complication include severe abdominal pain, fever, heavy rectal bleeding, or dizziness in the hours or days after the procedure. These are rare, but they require immediate medical attention.
How Often You Need One
The US Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for people at average risk. If you choose colonoscopy as your screening method and the results are normal (no polyps, no abnormalities), you won’t need another one for 10 years. If polyps are found, your doctor will recommend a shorter interval, typically 3 to 5 years, depending on the number, size, and type of polyps removed.
People with a family history of colorectal cancer, inflammatory bowel disease, or certain genetic conditions often need to start screening earlier and repeat it more frequently.