Always consult your doctor before taking any action that could affect your health.
"Selenium is a mineral that is being considered a crucial antioxidant which lowers your chance of getting Prostate cancer significanty. Countries like Canada whose soils have low levels of selenium also have higher rates of cancer including prostate cancer. A study done in the USA more than 14 years ago strongly supports the selenium-prostate cancer link. In a random group of men with skin cancer, half were given selenium and half were given a placebo. Although scientists found hat the mineral didn't decrease skin cancer recurrence, they discovered a 70% reduction in prostate cancer in men taking selenium! Since then, other researchers are finding similar results.
Caution: doses higher that 200 mcg of selenium a day may be toxic and cause extra health problems". Brazil Nuts (the food) has the highest content of selenium and it is always better to get your vitamins and minerals naturally.
Source: "Eating Right for Life - Prostate Cancer Nutrition & you" ISBN O-9698972-8-6
http://www.cnn.com/HEALTH/men/9906/17/selenium.prostate/index.html