Millions of people have endured a colonoscopy, believing the dreaded exam may help keep them from dying of colon cancer. For the first time, a major study offers clear evidence that it does.

Removing precancerous growths spotted during the test can cut the risk of dying from colon cancer in half, the study suggests. Doctors have long assumed a benefit, but until now research hasn’t shown that removing polyps would improve survival – the key measure of any cancer screening’s worth.

Some people skip the test because of the unpleasant steps need to get ready for it.

A second study in Europe found that colonoscopies did a better job of finding polyps than another common screening tool – tests that look for blood in stool. Both studies were published in Thursday’s New England Journal of Medicine.

Colorectal cancer is the second-leading cause of cancer death in the U.S. and the fourth worldwide. More than 143,000 new cases of cancers of the colon or rectum are expected in the U.S. this year and nearly 52,000 people will die from it, according to the American Cancer Society.

People of average risk of colon cancer ages 50 to 75 should get screened, but only about half in the U.S. do.

In a colonoscopy, a thin, flexible tube with a tiny camera is guided through the large intestine. Growths can be snipped off and checked for cancer. Patients are sedated, but many dread the test because it requires patients to eat a modified diet and drink solutions the previous day to clear the bowels.

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