Many factors influence toxicity. Supplements would not be on the market if they weren't safe. The FDA cannot limit the quantity or concentration of nutrients that a single pill can contain. The only exception is folacin. Excesses of one nutrient may cause nutritional imbalances or increase the need for other nutrients.
Some supplements interfere with the action of medications, creating a variety of ill effects.
National food consumption data and dietary surveys show that the majority of Americans get the nutrients they need through food alone. Some nutrients, like calcium and iron, may require careful food selection but can be obtained from adequate amounts of foods. Certain individuals have special needs and may benefit from taking a supplement. Some vegetarians, especially strict vegans who eat no meat, dairy products or eggs, may not receive adequate amounts of iron, calcium, zinc and vitamin B-12. Individuals with certain diseases or physiological conditions may require supplementation.