Selenium is a mineral found in the soil. Selenium naturally appears in the water and some foods. While people only need a very small amount, selenium plays a key role in metabolism. Selenium has attracted attention because of its antioxidant properties. Antioxidants protect cells from damage. Evidence that selenium supplements may reduce the odds of prostate cancer has been mixed, but most studies suggest there is no real benefit. Selenium does not seem to affect the risk of colorectal or lung cancer. Selenium has also been studied for the treatment of dozens of conditions. They range from asthma to arthritis to dandruff to infertility. However, the results have been inconclusive.