In our quest to fight colds, build muscle or have smoother skin, some of us believe that taking extra doses of vitamins and minerals is the best way to go. But taking large doses of any one supplement can do us more harm than good, experts say.

"Unless you are suffering from a severe deficiency, if you have a disease or disorder that is causing you to be nutrient-deficient, the most you are going to need is a high-quality multivitamin supplement, to be taken as insurance, and not used as your main source of nutrients," says Laurie Tansman, a clinical nutritionist at Mount Sinai Medical Center in New York.

Most people won't get into trouble taking a high-potency multivitamin. But problems can occur, experts say, when you start playing around with individual doses—even when recommended by some diet and fitness programs.


Vitamins A, D and E are among those causing the most concern because they are fat-soluble, can be stored for long periods of time in the body-mostly in the liver-and a cumulative buildup can occur turning your vitamin regimen from healthy to toxic.

  • Vitamin D is one of the most toxic supplements, and in very large amounts canlead to liver and kidney failure, says Jyni Holland, a clinical nutritionist at New York University Medical Center.
  • Take too much vitamin A and you could end up with hair loss, nausea, vomiting and significant joint pain. And there is evidence to show that too much vitamin A may cause birth defects, so it is recommended that pregnant women take no more than what comes in their prenatal vitamins.
  • Too much vitamin E can thin the blood so much it could lead to internal hemorrhaging, particularly if you are taking any blood-thinning medication for a cardiac or hypertension problem.
  • Vitamin K has the reverse effect—causing blood to clot. However it, too, can cause problems when used by people who are taking blood-thinning drugs.
  • The B vitamins and vitamin C are water-soluble and therefore don't build up in tissues and rarely reach toxic levels on their own. They are considered relatively safe, even in high doses-unless you are also taking iron supplements.

Because vitamin C enhances iron absorption, taking large doses of both could increase your risk of a toxic reaction.

In very high levels, vitamin B-6 has been associated with neurological symptoms such as nerve tingling, while vitamin B-3—also known as niacin—could be a problem if you suffer from heart disease.


  • Calcium has been linked to the formation of kidney stones and it can be packed with vitamin D, which helps the body absorb calcium. "If you are megadosing on calcium, you may also be megadosing on vitamin D and setting yourself up for some toxic reactions without even realizing you are doing so," Tansman says.
  • Iron supplements are commonly used by menstruating women, andare also sometimes used by body builders and athletes who are seeking to fight fatigue. "What most people don't realize is iron is an oxidant. And when it's exposed to oxygen inside the body, it becomes a free radical," Holland says. Free radicals cause an oxidation process that eventually damages cells and can increase the risk of certain diseases, including cancer.

Both Tansman and Holland agree that overdosing on supplements can be easier than you think, and that the best way to get all the nutrients you need is to take one multivitamin daily and eat plenty of vitamin-rich foods.

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