While vitamins are important in maintaining health, excessive levels of vitamins can lead to vitamin poisoning (hypervitaminosis).

Some of the vitamins most commonly linked to vitamin poisoning are Vitamin A, Vitamin B3, Vitamin B6 and Vitamin D. The medical names of the different kinds of excess are named after the vitamin; an excess of Vitamin A, for example, is called "hypervitaminosis A".

Excess doses of mineral supplements can also lead to poisoning. Hypervitaminosis, or mineral supplement poisoning, or both, can occur due to excessive intake of multivitamin supplements.

Vitamin C has been used in clinical trials in dosages over 100,000 mg, over 1000 times the daily recommended intake without ill effects.

What is the health risk of too much vitamin A? Hypervitaminosis A refers to high storage levels of vitamin A in the body that can lead to toxic symptoms. There are three major adverse effects of hypervitaminosis A:

   * birth defects,
   * liver abnormalities,
   * reduced bone mineral density that may result in osteoporosis (1)

Toxic symptoms can also arise after consuming very large amounts of preformed vitamin A over a short period of time.

Signs of acute toxicity include nausea and vomiting, headache, dizziness, blurred vision, and muscular uncoordination (1, 7-9, 41, 42).

Although hypervitaminosis A can occur when very large amounts of liver are regularly consumed, most cases of vitamin A toxicity result from an excess intake of vitamin A in supplements.

The Institute of Medicine has established Daily Tolerable Upper Levels (UL) of intake for vitamin A from supplements that apply to healthy populations (1). The UL was established to help prevent the risk of vitamin A toxicity. The risk of adverse health effects increases at intakes greater than the UL. The UL does not apply to malnourished individuals receiving vitamin A either periodically or through fortification programs as a means of preventing deficiency. It also does not apply to individuals being treated with vitamin A by medical doctors for diseases such as retinitis pigmentosa.

source: http://ods.od.nih.gov/factsheets/cc/vita.html#risk