Colonoscopy is the most accurate test for cancer of the colon and rectum, proven to detect the disease early and save lives. But even a very good test can be done too often. Here’s when you need it, and when you might not.

Having a colonoscopy more than once every five or ten years usually isn’t necessary.

A grape-like growth, or polyp, in the colon or rectum is common in adults and usually harmless. But some polyps—known as adenomas— may eventually turn into cancer. Health care providers can spot and remove polyps during a colonoscopy, which uses a flexible, lighted tube to examine the colon and rectum. If the test doesn’t find adenomas or cancer and you don’t have risk factors for the disease, your chance of developing it is low for the next ten years. That’s because the test misses very few adenomas, and colorectal cancer grows slowly. Even if one or two small, low-risk adenomas are removed, you’re unlikely to develop cancer for at least five years, and repeating the test sooner provides little benefit. So most people need the exam just once a decade, and only a few with larger, more serious polyps may need it more often than every five years.

Colonoscopy is a safe procedure. But occasionally it can cause heavy bleeding, tears in the colon, inflammation or infection of pouches in the colon known as diverticulitis, severe abdominal pain, and problems in people with heart or blood- vessel disease. Some complications can lead to blood transfusions, surgery, hospitalization, or rarely, death. The test also has inconveniences. You have to restrict your diet and take laxatives beforehand. And because the exam requires sedation, someone has to drive you home and you may miss a day of work. So you don’t want to have the test more often than necessary.

So when is it warranted?

Colon cancer screening should begin at age 50 for most people. If a colonoscopy doesn’t find adenomas or cancer and you don’t have risk factors, the next test should be in ten years. If one or two small, low-risk adenomas are removed, the exam should be repeated in five to ten years. Ask your health care provider when and how often to have a colonoscopy if you have inflammatory bowel disease; a history of multiple, large, or high-risk adenomas; or a parent, sibling, or child who had colorectal cancer or adenomas. Routine checks usually aren’t needed after age 75.

The following steps can help:

Make lifestyle changes. Eat more fruits, vegetables, and whole grains, and less fatty foods and red or processed meat. Lose excess weight, exercise, limit alcohol, and don’t smoke.

Get accurate test results. Carefully follow your health care provider’s instructions preparing your bowels before the procedure. If you have questions, call the office and go over them with the nurse.

Consider alternatives. If you’re at average risk, talk with your health care provider about other test options, and ask your insurer about coverage. Other tests that can find polyps and cancer, and require bowel prep, include flexible sigmoidoscopy, which uses a short tube to examine the rectum and lower colon, and CT colonography, in which a tube is inserted into the rectum and an X-ray scanner creates pictures. Stool tests can find signs of cancer and don’t require bowel preparation. Abnormalities found on an alternative test must be followed up with a colonoscopy.

Report warning signs. Those include changes in bowel habits lasting a week or two, such as rectal bleeding, dark or narrow stools, constipation or diarrhea, abdominal cramps, or the urge to move your bowels when you don’t need to. Constant fatigue, anemia, and unexplained weight loss can be later signs.