"More than half of Americans take some kind of vitamin supplement, and the most commonly taken is a multivitamin," Dr. John Michael Gaziano, chief of the Division of Aging at Brigham and Women's Hospital and a researcher at VA Boston, said in a statement. "No one has ever done a long-term trial to determine the potential health benefits or downsides of taking a multivitamin for a long period of time."
Gaziano and colleagues investigated the long-term effects of daily multivitamin use from data from the Physicians' Health Study II, which included 14,641 male physicians age 50 or older.
The researchers randomly assigned participants to a multivitamin or no multivitamin from 1997 to 2011. During the median follow-up of 11.2 years, researchers recorded 2,669 cases of cancer, including 1,373 prostate cancer cases and 210 colorectal cancer cases.
The study, presented at the annual American Association for Cancer Research International Conference on Frontiers in Cancer Prevention Research and published in the Journal of the American Medical Association, found an 8 percent reduction in total cancer occurrence among participants assigned to multivitamin use.
"We also saw trends for some of the major site-specific cancers, though the numbers were small and not significant," Gaziano said. "There also seemed to be a greater effect in people with previous cancer."