Selenium is an essential mineral, but you need only a small amount of it in your daily diet. Like all minerals, selenium is toxic if you take it in the form of a dietary supplement in excess doses. You can even experience symptoms of chronic selenium toxicity, or selenosis, if you take only a small amount for a long period of time. Selenium toxicity can even be fatal, the Linus Pauling Institute says.
The mineral selenium works together with vitamin E as an antioxidant, a substance that protects your cells from the harmful effects of free radicals. It's also important for healthy thyroid and immune system function. The recommended dietary allowance, or RDA, for selenium for adults age 19 and older is 55 mcicrograms a day. According to the Linus Pauling Institute, Americans get 80 to 110 micrograms of selenium a day from the foods they eat, on average. Selenium is found in seafood, red meat and food grown in selenium-rich roil. One ounce of Brazil nuts can hold more than 500 micrograms of selenium.
Supplemental selenium can be used therapeutically to address selenium deficiency. Solid evidence suggests that it's effective for this purpose. Selenium supplements, when used in conjunction with thyroid hormone replacement, might also be beneficial for people with Hashimoto's thyroiditis. Less evidence supports its use as a way to prevent cancer, heart disease and other medical conditions. According to the Office of Dietary Supplements, selenium toxicity rarely occurs in the U.S.; some documented cases were the result of the manufacturer accidentally making its supplement too strong.
The tolerable upper intake level, or UL, for selenium is 400 micrograms a day for adults; this includes the selenium you get from your daily diet. Supplemental selenium in excess of 100 micrograms can be harmful to your health, according to the Merck Manuals Online Medical Library. Early signs of selenium toxicity are a garlicky odor on your breath and a metallic taste in the mouth, according to the National Institutes of Health Office of Dietary Supplements. As toxicity progresses you'll likely notice fast hair loss and brittle nails, as well as other symptoms of selenosis such as nausea, vomiting, diarrhea, tiredness, irritability and skin rash.. Selenium toxicity can also cause nerve damage. Selenium toxicity is not just attributed to taking high doses -- it can with long-term use, explains the Linus Pauling Institute. If you develop any symptoms of selenium toxicity, discontinue use and speak to your doctor for a diagnosis.
Selenium supplements might not mix well with some medications. Some of these include blood thinners, barbiturates, chemotherapy drugs and drugs that lower cholesterol. Selenium supplements might not be appropriate if you are pregnant, breastfeeding or getting ready to have surgery, according to MedlinePlus.com. Most people get more than enough of this mineral from their diet and do not require supplemental selenium. Never self-treat with selenium supplements without first consulting with your treating physician.