The National Cancer Institute has stopped a major clinical trial studying whether vitamin E and selenium can prevent prostate cancer because early analysis suggests the supplements don't work. What's more, there is some evidence that they may actually be harmful.

The study is known as the SELECT trial. More than 35,000 men over age 50 in the US, Puerto Rico, and Canada were enrolled. Each participant was randomly assigned to take either selenium, vitamin E, a combination of the two, or a placebo for the course of the study.

After an average of five years, the analysis found no lower risk of prostate cancer in men taking the supplements, either alone or together. Men who were taking only vitamin E actually had a slightly higher risk of developing prostate cancer. And men taking only selenium seemed to have a slightly higher risk of developing diabetes. This increased risk was not statistically significant in either case. That means the findings could be due to chance; they don't prove the supplements raise prostate cancer and diabetes risk. Still, researchers called the findings "concerning."

Men in the study will get letters telling them to stop taking their study pills. The researchers plan to continue monitoring the participants for about 3 more years to see if there are long-term effects (good or bad) from taking the supplements.

The SELECT trial began in 2001, after other studies found a lower risk of prostate cancer in men taking either selenium or vitamin E. However, those other studies had not been designed to look at prostate cancer specifically; one had been studying lung cancer and the other skin cancer. The SELECT trial was designed to learn for certain whether vitamin E and selenium could prevent the prostate cancer.

For more information about the SELECT trial and what participants should do now, consult the National Cancer Institute's Questions and Answers: Selenium and Vitamin E Cancer Prevention Trial (SELECT).