Selenium is safe at the level people typically supplement (100200 mcg); however, taking more than 900 mcg of selenium per day has been reported to cause adverse effects in some people.⁵ Selenium toxicity can result in loss of fingernails, skin rash, and changes in the nervous system. In the presence of iodine-deficiency-induced goiter, selenium supplementation has been reported to exacerbate low thyroid function.⁶ Although most research suggests that selenium prevents cancer, one study found an increased risk of a type of skin cancer (squamous cell carcinoma) in people taking selenium supplements.⁷ The National Academy of Sciences recommends that selenium intake not exceed 400 mcg per day, unless the higher intake is monitored by a healthcare professional.⁸ In a double-blind study of people who took 200 mcg of selenium per day for several years to prevent recurrences of skin cancer, the incidence of diabetes was higher in people who received selenium (9.7%) than in those who received a placebo (6.5%).⁹ While this difference was statistically significant, this finding should be considered preliminary, since the study was not originally designed to test whether selenium influences the risk of developing diabetes.