Men should avoid taking vitamin E or selenium supplements according to U.S. researchers who say the pills — which are often taken for cancer prevention — actually increased the risk of aggressive prostate cancer in some cases.

The study used data from the Selenium and Vitamin E Cancer Prevention Trial, or SELECT, a randomized, placebo-controlled trial of more than 35,000 men in the U.S.

Men using these vitamin E and selenium supplements should stop, a researcher says. (David Gray/Reuters)

The trial was stopped earlier than planned because it showed no protective effect for selenium and suggested an increased risk of cancer from vitamin E. Researchers told the men in the study to stop taking the supplements and kept checking in with them.

Investigators then re-analyzed the data for 1,739 men from the trial who were previously diagnosed with prostate cancer, compared with 3,117 others, without prostate cancer, who were matched to the patients by race and age.

The focus of the research then shifted to how the amount of selenium in the men's bodies before they started taking supplements affected cancer rates.

The bottom line was there were no benefits to any men from either selenium or vitamin E supplements, said researchers. Taking selenium supplements increased the risk of high-grade cancer among men who had naturally high selenium levels.

"Men using these supplements should stop, period," study author Alan Kristal of the Public Health Sciences division at Fred Hutchinson Cancer Research Center in Seattle said in a release.

Neither selenium nor vitamin E supplementation offers any known benefits, only risks, he said.

"While there appear to be no risks from taking a standard multivitamin, the effects of high-dose single supplements are unpredictable, complex and often harmful."

There are still open questions about selenium and prostate cancer risk, according to a journal editorial published with the study. For example, animal studies suggest the form of selenium matters, such as whether it comes from pills, enriched wheat and broccoli, or yeast.

"We need to revisit the question of evaluating the prevention of a single disease in healthy people, where competing risks may obscure reality. Even if a subset can be identified that would benefit from some form of selenium or vitamin E supplementation, it must not be only a benefit to a single gland or based on one disease," biostatistician Paul Frankel from City of Hope Comprehensive Cancer Center in Duarte, Calif. concluded in the editorial.

The U.S. National Cancer Institute funded the study.