At least six major studies are underway to test selenium supplements' ability to prevent prostate cancer and/or slow the growth of existing tumors. Researchers are looking to confirm a 1994 Arizona study in which men who took 200 micrograms of the mineral daily for ten years cut their prostate cancer risk by 63%. While selenium is found in meat, grains, and dairy products, it is difficult to tell how much is in a person's diet because the selenium content of a specific food item varies in accordance with where the item was grown. Since a man's diet may be lacking in selenium, a daily supplement of 200 micrograms is a reasonable choice for men concerned about their prostate cancer. Selenium supplementation should be limited to 200 micrograms daily because, at very high doses, the mineral is toxic.