Although the margin of safety between the needed level and the toxic level is quite wide for some nutrients, Vanderveen says, it is very narrow for others, such as the mineral selenium. The agency believes that problems with nutrients can be best handled by subjecting them to the same labeling requirements as other foods. As few as six high-potency iron tablets could seriously injure a child weighing 22 pounds or less. Iron is the most common cause of U.S. child poisoning deaths, causing about 2,000 poisonings a year, mainly in young children taking their mothers' prenatal supplements. Toxicity depends on the chemical form and the selenium content of the foods eaten and has occurred with high intakes after a few weeks.
High intakes can cause bone pain, muscle weakness, numbness, or other symptoms of nerve disorder.
Continuous high intakes can cause kidney damage and bone deformity. Just because there haven't been a lot of reports of adverse effects associated with use of dietary supplements doesn't necessarily mean they're safe. A person taking a supplement may not associate an adverse effect with it, especially if the individual also is taking one or more medications or is chronically ill. For example, muscle weakness or numbness caused by too much vitamin B6 might be ignored as a sign of conditions associated with aging. Or adverse effects may take time to develop, so that it's difficult to link them with a specific supplement.