Selenium is safe at the level people typically supplement (100–200 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 goitre, 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 a few 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.