Merck Manual

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Vitamin C Excess

(Vitamin C Toxicity)


Larry E. Johnson

, MD, PhD,

  • University of Arkansas for Medical Sciences
  • Central Arkansas Veterans Healthcare System

Last full review/revision Nov 2020| Content last modified Nov 2020
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Vitamin C toxicity is very rare. Some people take high doses of vitamin C because it is an antioxidant.

Vitamin C (ascorbic acid) is essential for the formation, growth, and repair of bone, skin, and connective tissue (which binds other tissues and organs together and includes tendons, ligaments, and blood vessels). It is also essential for the normal function of blood vessels. Vitamin C helps maintain healthy teeth and gums. It helps the body absorb iron, which is needed to make red blood cells. Vitamin C also helps burns and wounds heal. Good sources of vitamin C include citrus fruits, tomatoes, potatoes, broccoli, strawberries, and sweet peppers. (See also Overview of Vitamins.)

Like vitamin E, vitamin C is an antioxidant: It protects cells against damage by free radicals, which are by-products of normal cell activity and which participate in chemical reactions within cells. Some of these reactions can cause damage over a person's lifetime.

Some people take high doses of vitamin C because it is an antioxidant, which protects cells against damage by free radicals. Free radicals are thought to contribute to many disorders, such as atherosclerosis, cancer, lung disorders, the common cold, and memory loss. However, whether taking high doses of vitamin C protects against or has any beneficial effect on these disorders is unclear; most studies have shown no benefit. Evidence of a protective effect against cataracts is strongest.

High doses (up to the safe upper limit—2,000 milligrams a day) of vitamin C are usually not toxic to healthy adults. Occasionally, higher doses cause nausea or diarrhea and interfere with the balance of antioxidant activity in the body.

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