Part of the problem with vitamins A and D is that they are fat soluble and large doses can build up in body tissue. Most vitamins, by contrast, are water soluble, and large doses are simply passed out of the body through the urine.
"Vitamin A can easily be toxic," says Sporny. "Just two to three times the RDA can increase the risks of hip fracture and birth defects. And now, there's some debate that the amount of vitamin A found in a multi-vitamin might even be too much, especially for women trying to conceive and for an older population."
Like many nutrients, the amount of vitamin D needed varies with age, and people over 70 may need more. "Taking some vitamin D is OK for an aging population," says Sporny, "but too much vitamin D can cause damage to blood vessels, eye tissue and kidneys."
Though vitamin B6 is water soluble, medical experts have found excessive doses from vitamin supplements often cause nerve damage.
So are vitamin and mineral supplements dangerous? Don't go emptying your medicine cabinet just yet.
"In moderation, most vitamin and mineral supplements are perfectly safe," says Rusyniak, "but care should be exercised when taking multiple supplements containing the same vitamins and minerals."
Sporny also advises a common-sense approach. "A basic multi-vitamin and mineral supplement, specific to your age and sex group, can be a good addition to your diet. But taking vitamin supplements willy-nilly without any medical supervision can exacerbate problems."