Selenium supplements are sometimes marketed as antioxidants, although a research study with 100 micrograms selenium indicated taking antioxidant supplements doesn't seem to offer any health benefits.
Selenosis (having too much selenium in your body) results in gastrointestinal symptoms, hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage. The Institute of Medicine of the National Academy of Sciences set a tolerable upper dietary intake level for selenium at 400 micrograms per day for adults.
Your body requires about 55 micrograms of selenium each day. You can get that amount of selenium from your diet when you eat whole grains, nuts, and seafood. Brazil nuts are particularly rich in selenium.
Even if you eat a lot of these foods it would be difficult to consume too much selenium from your diet. Selenium toxicity usually occurs from industrial exposure to selenium and not by taking dietary supplements, but it is possible to ingest too much selenium if you regularly take more than 900 micrograms per day.
Speak with your doctor before taking large doses of any dietary supplement, including selenium, and follow the directions on the product label.
Briançon S, Boini S, Bertrais S, Guillemin F, Galan P, Hercberg S. "Long-term antioxidant supplementation has no effect on health-related quality of life: the randomized, double-blind, placebo-controlled, primary prevention SU.VI.MAX trial." Int J Epidemiol. 2011 Dec;40(6):1605-16.
Office of Dietary Supplements of the National Institutes of Health "Dietary Supplement Fact Sheet: Selenium." Accessed February 9, 2012. http://ods.od.nih.gov/factsheets/selenium.
Yang GQ, Zhou RH. "Further observations on the human maximum safe dietary selenium intake in a seleniferous area of China." J Trace Elem Electrolytes Hlth Dis 1994;8:159-65.