Selenium supplements are available in three forms - capsules, tablets, and extended-release tablets. Extended-release tablets are designed to keep a steady amount of the substance in the blood by releasing it slowly as the tablet passes through the digestive system. You should take the supplement as your doctor directs or follow the instructions on the label carefully. The typical dose varies depending on several factors, but will probably range from 40 to 70 micrograms (mcg) per day for men and 45 to 55 mcg per day for women.If you miss a scheduled dose, do not double up doses. Just take the missed dose as soon as possible, but not if it is too close to your scheduled dose.