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Vitamins play a key role in your health. Their functions range from supporting cell growth and promoting tissue strength to producing energy. They're an essential part of your diet because your body can't make adequate amounts of vitamins on its own. But getting too much of a few specific vitamins can negatively affect your health. Fat-soluble vitamins, like vitamins A and D, are generally more toxic because they're stored in your body's tissues. Water-soluble vitamins are not stored in your body, but some of them -- like vitamin C and some B vitamins -- can cause toxicity too.

Vitamin A

Vitamin A is important in forming light receptors in the eye and maintaining cells on the surface of tissues throughout the body. Vitamin A toxicity may occur not only when large doses are taken by mouth but also when medications containing certain forms of vitamin A -- such as isotretinoin -- are used in moderately high doses to treat skin conditions. Vitamin A toxicity causes headaches, rashes, coarse hair, hair loss, dry or peeling skin and cracked lips. It also increases the risk of osteoporosis -- thin bones -- and may rarely cause pseudotumor cerebri -- increased pressure in the head characterized by such symptoms as headaches, nausea and vision changes. Birth defects may occur when isotretinoin is used during pregnancy and excessive amounts of any form of vitamin A should probably be avoided in women who are pregnant.

Vitamin D

Vitamin D has numerous effects, including regulating the body's calcium levels by increasing the absorption of calcium from the gut. Taking large doses of vitamin D supplements can cause toxicity, called hypervitaminosis D. The main effect of this is hypercalcemia -- high calcium levels. Symptoms of hypercalcemia include nausea, loss of appetite, excessive urination, itchiness and weakness. High calcium levels can also cause kidney stones and affect mental function, causing nervousness, anxiety or confusion.

Vitamin C

Vitamin C is necessary for the formation of collagen, which is the main protein in skin, ligaments and blood vessel. It also helps the immune system and is a potent antioxidant, which means that it's able to neutralize molecules called free radicals that can damage and even destroy cells. Large doses of vitamin C are generally safe, although they can cause nausea, diarrhea or abdominal cramps.

Vitamin B6

Vitamin B6 plays an important role in blood, skin and nervous system metabolism. Large doses of vitamin B6 supplements for months can damage the nerves in the arms and legs, causing difficulties with coordination. Nausea, heartburn, rashes and increased sensitivity to the sun may also occur. Additionally, there may be an increased risk of birth defects in children whose mothers take vitamin B6 during the first part of pregnancy, although this issue is somewhat controversial according to the National Institutes of Health Office of Dietary Supplements.


Vitamin B3, more commonly known as niacin, helps the body process sugars, fats and other substances. It is also used to treat high cholesterol and triglycerides -- fat circulating in the blood. Excessive intake often produces redness, warmth and burning of the skin known as a niacin flush. High levels of niacin can also cause vision changes, dizziness, joint pain due to an acute attack of gout, abdominal pain and palpitations -- feeling the heart beating hard, fast or irregularly. Rarely, niacin can lead to liver damage, particularly if the sustained-release form is used. This damage increases blood levels of bilirubin, which makes the skin and whites of the eyes develop a yellow tint.

When to Seek Medical Attention

Always check with your doctor before you start taking vitamin supplements to minimize your risk of toxicity. If you take vitamin supplements and notice any symptoms of toxicity, stop taking them and let your doctor know immediately.