WEDNESDAY, Oct. 17 (HealthDay News) -- What has become a daily ritual for millions of Americans may have an unintended and beneficial effect.

A new study finds that men who take multivitamins every day for several years may lower their risk of cancer by a small amount.

"Total cancer was reduced modestly by about 8 percent," said Dr. J. Michael Gaziano, lead author of the study that appears online Oct. 17 in the Journal of the American Medical Association(JAMA).

However, the study found no difference in death rates from cancer between people who took vitamins and those who took placebo pills.

There was "a suggestion of a benefit in cancer mortality of about 12 percent, but that was not quite statistically significant," added Gaziano, who is a researcher with Brigham and Women's Hospital and the Veterans Administration Boston Healthcare System.

The findings are also being presented Wednesday at the annual cancer prevention conference of the American Association for Cancer Research, held in Anaheim, Calif.

According to study background information, at least one-third of American adults take multivitamins.

Although these tablets were originally developed to prevent vitamin and mineral deficiencies, many people now also take multivitamins in the hopes that they might prevent various diseases, despite mixed evidence on the subject.

Some studies have shown potential harm from high doses of certain single vitamins.

For this study, about 14,700 male physicians, all at least 50 years old, were randomly assigned to take either one multivitamin a day, or a placebo.

The multivitamin used here was Centrum Silver. The study was funded by the U.S. National Institutes of Health and BASF Corp. Multivitamins were supplied by Pfizer, which makes Centrum Silver.

At the beginning of the study, 1,300 men (9 percent) of the doctors had a history of cancer, although they were considered cured when they enrolled in the trial.

After an average treatment time of 11.2 years, researchers found an overall 8 percent reduction among men who took the multivitamin daily.

There was no apparent effect on prostate cancer, which, not surprisingly, was the most common cancer among this group of middle-aged and older men.

Researchers also saw some improvements in risk for bladder, lung and colorectal cancer and in men who had had a previous cancer, but these numbers were not statistically significant.

There seemed to be no adverse side effects from the multivitamins, the researchers said.

No one knows why multivitamins may have this beneficial effect, although they are designed to mimic the content of fruits and vegetables, which have been linked with a reduced risk of cancer, said Gaziano.

"We can't speculate on whether there was one [vitamin or mineral] particularly amongst the many things that are in a standard multivitamin and mineral supplement or whether it was just the combination given at the right dose," said Gaziano, who is also a contributing editor for JAMA.

The study also can't be extrapolated to women or men of different ages or with different types of vitamin supplements.

But the main reason to take multivitamins -- which is to prevent vitamin and mineral deficiencies -- is unchanged by this paper, said Dr. David Weinberg, chief of the department of medicine at Fox Chase Cancer Center in Philadelphia.

"It turns out that there might be some modest benefit and there does not appear to be any significant harm associated with multivitamins when it comes to cancer prevention," he added. "This data adds maybe a small, additional impetus to people to say 'Not only might multivitamins help make sure my nutritional status is as good as it can be, but they might also help prevent cancer in general.'"

The authors also collected information on multivitamin use and heart disease, eye disease and cognitive function, which will be presented at later dates.

More information

The U.S. National Cancer Institutehas more on vitamins as well as minerals and herbs in cancer treatment.