A colonoscopy can show polyps, colorectal cancer, diverticulosis, hemorrhoids, signs of inflammatory bowel disease, infections, and other structural changes in the lining of your colon and rectum. It’s one of the few procedures that is both diagnostic and therapeutic, meaning your doctor can spot a problem and often treat it during the same visit. For average-risk adults, the U.S. Preventive Services Task Force recommends starting screening colonoscopies at age 45, repeated every 10 years.
Polyps and Precancerous Growths
The most common finding on a colonoscopy is a colon polyp, a small growth on the inner lining of the colon. Polyps come in different shapes. A pedunculated polyp looks like a mushroom, with a distinct head and stalk. A sessile polyp has no stalk and sits flat against the colon wall like a raised bump. The shape matters because it affects how the polyp is removed and how likely it is to cause problems later.
Not all polyps are dangerous. Hyperplastic polyps are extremely common and carry no significant risk of becoming cancerous. Adenomatous polyps, on the other hand, are the ones that can eventually develop into colorectal cancer if left in place. Your doctor won’t know which type a polyp is just by looking at it, so nearly all polyps found during a colonoscopy are removed on the spot and sent to a lab for analysis. For polyps smaller than 10 mm, the doctor typically uses a small wire loop called a cold snare to clip them off painlessly. Larger or stalked polyps may require a heated snare or more advanced techniques.
The pathology report you receive a week or two later will tell you whether the polyp was harmless or adenomatous, and that result determines when you’ll need your next colonoscopy. A few small hyperplastic polyps might mean you can wait the full 10 years. Multiple adenomatous polyps could shorten that interval to three to five years.
Colorectal Cancer
Colonoscopy is the gold standard for detecting colorectal cancer. Tumors appear as irregular masses, ulcerated areas, or regions where the colon wall looks abnormally thickened. When a suspicious area is found, the doctor takes small tissue samples (biopsies) from multiple spots so a pathologist can confirm whether cancer cells are present and, if so, how abnormal they look. Catching cancer at an early stage, before it has grown through the colon wall or spread to lymph nodes, dramatically improves survival. That’s the entire rationale behind routine screening: finding and removing precancerous polyps before they ever become malignant.
Diverticulosis
Between 40% and 60% of people have diverticula, small pouches that form in the muscular wall of the colon and bulge outward. They become more common with age and tend to cluster in the sigmoid colon, the S-shaped section just above the rectum. The condition of simply having these pouches is called diverticulosis, and it usually causes no symptoms at all. Many people first learn they have it from an incidental finding on a screening colonoscopy.
Diverticulosis itself doesn’t require treatment, but your doctor will note it because the pouches can occasionally become inflamed or infected (diverticulitis) or bleed. If you know you have diverticula, you can watch for warning signs like sudden lower abdominal pain or rectal bleeding and seek care earlier.
Internal Hemorrhoids
Everyone has cushion-like clusters of veins in the lining of the lower rectum and anus. These structures help prevent stool leakage. When they swell, they become what we call hemorrhoids. Internal hemorrhoids sit inside the lower rectum, where there are no pain-sensing nerves, so you typically won’t feel them. They can, however, cause bright red blood in the toilet. A colonoscopy can identify swollen internal hemorrhoids, grade their severity, and rule out more concerning causes of rectal bleeding at the same time.
Inflammatory Bowel Disease
A colonoscopy can reveal the characteristic patterns of Crohn’s disease and ulcerative colitis. In ulcerative colitis, the inflammation typically starts at the rectum and extends continuously up the colon, with redness, swelling, and shallow ulcers visible on the surface. Crohn’s disease tends to cause patchy inflammation with deeper ulcers and can affect any part of the digestive tract. Biopsies taken during the procedure help confirm which condition is present and how active the inflammation is. For people already diagnosed with inflammatory bowel disease, follow-up colonoscopies monitor how well treatment is working and screen for dysplasia, a precancerous change that becomes more likely the longer the disease has been active.
Vascular Abnormalities
The scope can also reveal angiodysplasia, clusters of fragile, abnormally dilated blood vessels in the colon wall. These are more common in older adults and can be a hidden source of chronic blood loss or iron-deficiency anemia. They appear as small, bright red spots during the exam and can often be treated on the spot with heat or a small clip to stop or prevent bleeding.
Less Common Findings
Occasionally a colonoscopy turns up something unexpected. Melanosis coli is a harmless condition where the inner lining of the colon turns dark brown or black. It’s caused by a buildup of a pigment called lipofuscin, most often from long-term use of stimulant laxatives, though chronic diarrhea, inflammatory bowel disease, and even heavy NSAID use can contribute. The discoloration itself doesn’t need treatment and typically fades once the underlying cause is addressed.
Parasitic infections, strictures (narrowed sections of the colon from scarring or inflammation), and rare tumors like carcinoid growths can also be identified visually or through biopsies taken during the procedure.
What a Colonoscopy Can Miss
No screening test is perfect. Flat or sessile polyps, which sit flush against the colon wall, are missed at a significantly higher rate than polyps that protrude on a stalk. One study in Intestinal Research found a miss rate of about 33% for flat or sessile polyps compared to roughly 8% for pedunculated ones. That makes flat polyps nearly four times more likely to be overlooked. This is one reason bowel preparation matters so much: residual stool can hide these subtle lesions.
Doctors score your bowel preparation using a standardized scale that rates cleanliness from 0 (completely unprepared) to 9 (perfectly clean) across three segments of the colon. A score of 6 or higher is generally considered adequate, but detection rates improve substantially with better prep. If your preparation was poor, your doctor may recommend repeating the procedure sooner than the usual interval.
What to Expect During and After
A colonoscopy typically takes 30 to 60 minutes. You’ll receive sedation, so you’ll be drowsy or asleep during the procedure and won’t feel the scope or any polyp removals. Afterward, you’ll spend about an hour in a recovery area while the sedation wears off. You’ll need someone to drive you home, and most doctors advise against driving, working, or making important decisions for the rest of the day. Most people feel back to normal by the next morning.
If polyps were removed, you can expect your pathology results within one to two weeks. Those results, along with the number and size of any polyps found, will determine your recommended follow-up schedule. A completely clean colonoscopy with good bowel prep typically means you won’t need another one for 10 years.