LOS ANGELES – 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 any 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 research hasn’t shown before 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 needed to get ready for it.

“Sure, it’s a pain in the neck. People complain to me all the time, ‘It’s horrible. It’s terrible,’” said Dr. Sidney Winawer, a gastroenterologist at Memorial Sloan-Kettering Cancer Center in New York who helped lead the study. “But look at the alternative.”

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 today’s New England Journal of Medicine.

Colorectal cancer is the second leading cause of cancer death in the United States 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.