Millions of Americans take a daily multivitamin, hoping it will help keep them healthy, and now a large clinical trial has shown that it actually has a modest effect on preventing cancer — at least in older men.

The long-running Brigham and Women’s Hospital study involved nearly 15,000 male physicians over age 50 and found that those who took a Centrum Silver multivitamin, for an average of 11 years, had an 8 percent lower risk of getting cancer, compared with those randomly chosen to get placebos.