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Vitamin Overdose/Toxicity Basics

Virtually all over-the-counter antioxidant supplements have a unique blend of vitamins in them and generally include appropriate dosage labelling. Below is a description of vitamins and some of what is known about potential toxicity or adverse effects of high doses. There are other theoretical risks with taking high doses of antioxidants, such as acute allergic reactions and unexpected pharmacological interactions. People should ask their doctor about the specific vitamins and dosages they plan to take. Specifically, they can ask about the use of vitamins to help during times of particularly high stress and physical activity, about possible complications, about contraindications with any other medications or supplements they are taking, and about FDA guidelines and research into the risks and benefits of higher doses.


Adapted from MEDLINEplus Health Information, Health Watch (a Public Service of the Office of News and Publications & the Library at UT Southwestern Medical Center at Dallas), and Lifeforce Hospitals.

Vitamin A can build up in the system with successive doses and is known to cause health problems at high levels. Too much vitamin A (over 25,000 IU per day), for instance, can cause headaches, hair loss, and liver damage, or abnormal fetal development in pregnant women.

Vitamin E can also build up, but there are no published case reports of serious problems resulting from overdoses. People who take blood thinners have increased risk of bleeding because vitamin E can increase the action of blood thinning medications.

Large doses of Vitamin D (daily doses of 50,000 IUs, 125 times the U.S. RDA) can result in increased calcium absorption from the intestinal tract, and possibly also to increased calcium resorption from the bones, leading to elevated levels of calcium in the blood. This can lead to abnormal calcium deposition in soft tissues, such as the heart and lungs, reducing their ability to function.

There is no known toxicity to Thiamine (B1) or riboflavin (B2). Because riboflavin is a water-soluble vitamin, excess amounts are excreted by the body in the urine.

Large doses of Vitamin B6 (more than 400 mg a day, 200 times the RDA) can cause neurological disorders and numbness in the mouth and hands.

With Vitamin C, toxicity does not normally occur, since it is water soluble and is regularly excreted by the body. Recent studies have shown, however, that excessive doses of vitamin C (i.e., more than the RDA) can lead to toxicity. Taking more than 1-2 grams at a time can cause diarrhea and intestinal discomfort.

Toxicity from excessive Folic acid (B9) intake does not normally occur, as folic acid is water soluble and regularly excreted by the body. However, doses of folic acid that greatly exceed the RDA may obscure a serious condition called pernicious anemia.

Symptoms of overdose of Niacin (B3) include: whole flushing of the body, burning sensation in the eyes, ears, nose, and throat, blurred vision, skin itching, nausea, vomiting, abdominal pain, diarrhea, lightheadedness. A niacin overdose is usually not life threatening; however, for large overdoses, call Poison Control.

Daily doses of Iron higher than 100 mg (six times the RDA) could interfere with absorption of zinc, a mineral that speeds wound healing and helps regulate the immune system.

Alpha Lipoic Acid is well tolerated, but in sensitive people at high doses, some people experience skin reactions, nausea, & vomiting. These reactions tend to occur only at extreme doses (one clinical trial administered 1200 mg IV per day and some patients reported negative side effects). Normal supplement doses are between 50 and 500 mg per day.

Note: This list is not all inclusive. If vitamin combinations also contain minerals or other ingredients, these to could achieve toxic levels. When taken in extremely high doses, just about any vitamin can be dangerous. There hasn't been much evidence that megadoses of vitamins has much benefit.