After a series of conflicting reports about whether vitamin pills can stave off chronic disease, researchers announced on Wednesday that a large clinical trial of nearly 15,000 older male physicians followed for more than a decade found that those taking a daily multivitamin experienced 8 percent fewer cancers than the subjects taking dummy pills.

While many studies have focused on the effects of high doses of particular vitamins or minerals, like calcium and vitamin D, this clinical trial examined whether a common daily multivitamin had an effect on overall cancer risk. A randomized, double-blinded study of the kind considered the gold standard in medicine, the study was one of the largest and longest efforts to address questions about vitamin use.

The findings are to be presented Wednesday at an American Association for Cancer Research conference on cancer prevention in Anaheim, Calif., and the paper was published online in the Journal of the American Medical Association.

The reduction in total cancers was small but statistically significant, said the study’s lead author, Dr. J. Michael Gaziano, a cardiologist at Brigham and Women’s Hospital and the VA Boston Healthcare System. While the main reason to take a multivitamin is to prevent nutritional deficiencies, Dr. Gaziano said, “It certainly appears there is a modest reduction in the risk of cancer from a typical multivitamin.”

He noted that other measures are likely to protect against cancer more effectively than daily use of multivitamins.

“It would be a big mistake for people to go out and take a multivitamin instead of quitting smoking or doing other things that we have a higher suspicion play a bigger role, like eating a good diet and getting exercise. You’ve got to keep wearing your sunscreen,” Dr. Gaziano said.

The study was supported by the National Institutes of Health and a grant, initiated by the investigators, from the chemical company BASF Corporation. Pfizer provided the multivitamins. The sponsors did not have input into the study design, data analysis or manuscript preparation, the authors said.

About half of all Americans take some form of a vitamin supplement, and at least one third take a multivitamin. But many recent vitamin studies have been disappointing, finding not only a lack of benefit but even some harm associated with large doses of certain supplements. The 2010 dietary guidelines for Americans state there is no evidence to support taking a multivitamin or mineral supplement to prevent chronic disease.

The American Cancer Society recommends that people who take supplements choose a balanced multivitamin that contains no more than 100 percent of the daily value of most nutrients.

Though several researchers said they were somewhat surprised by the findings, others called the results encouraging.

“It is a small overall effect, but from a public health standpoint it could be of great importance,” said Dr. E. Robert Greenberg, an affiliate at Fred Hutchinson Cancer Research Center in Seattle. “Other than quitting smoking, there’s not much else out there that has shown it will reduce your cancer risk by nearly 10 percent.”

Multivitamin use had no effect on the incidence of prostate cancer, which was the most common cancer diagnosed in the study participants. When researchers looked at the effect of vitamin use on all other cancers, they found a 12 percent reduction in occurrence. Overall cancer deaths were reduced among vitamin users, but the difference was not statistically significant.

A major limitation of the study is that it only included male physicians, who were particularly healthy, with extremely low smoking rates, said Marji McCullough, a nutritional epidemiologist with the American Cancer Society. “We still need to find out whether these findings can be applied to others in the population,” she said.

While the research effort may have benefited from the fact that the physicians who participated were very diligent about taking their pills, the researchers also suggested that the effect of multivitamin use may have been muted because the participants were very health conscious to begin with.

Dr. David Chapin, 73, a gynecologist at Beth Israel Deaconess Medical Center in Boston who participated in the trial, said that although he has “never believed” in vitamins, he may start taking a daily multivitamin now, despite the modest benefit.

“A lot of studies make big news, but when you look at the nitty-gritty, they don’t show all that much,” Dr. Chapin said, adding that he recently discovered he had been taking a placebo pill. “This was a very reliable study, it was very well designed and administered, and it went on and on and on.”