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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. To be safe, the 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.
Selenium supplements are sometimes marketed as antioxidants, although a research study using antioxidant supplements including 100 micrograms selenium indicates taking antioxidant supplements doesn't seem to offer any health benefits. Speak with your doctor before taking large doses of any dietary supplement, including selenium, and follow the directions on the product label.
Sources:
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.
