Vitamin B12 Dosage Limits & Safety

Vitamin B12, also known as cobalamin, is one of the B complex vitamins, but it is unique among this family of nutrients in several respects. B12 is the only vitamin that contains an essential mineral, namely cobalt. Although vitamin B12 is water-soluble -- excess doses are eliminated in your urine -- it is the only B vitamin that is stored in your body for long periods of time, often for several years. B12 is absorbed differently from other B vitamins, and your daily requirements for this nutrient are much lower than for other B vitamins.

Functions

Vitamin B12 serves several important roles in your body. It is essential for manufacturing and maintaining myelin, which is the protective insulation that surrounds your nerve cells. In conjunction with folate, vitamin B12 participates in the production of hemoglobin, which is the oxygen-carrying protein in your red blood cells. B12 is also necessary for the regeneration of folate within your cells, thereby ensuring the continued synthesis of DNA and RNA for chromosomal production and repair.

Sources

Vitamin B12 is only found in appreciable amounts in animal-based foods, but even those foods contain relatively small quantities. Hence, large doses are unlikely to be acquired from your diet. Good sources of vitamin B12 include organ meats, such as liver, heart and kidney, red meats, fish, shellfish, egg yolks and milk products. Strict vegetarians have difficulty getting sufficient amounts of vitamin B12 from their usual diet, although tempeh and some sprouts may contain low levels of this nutrient.

Deficiency

According to the June 2008 "Food and Nutrition Bulletin," most cases of vitamin B12 deficiency are due to insufficient intake or poor absorption of the vitamin from foods. Intestinal absorption of cobalamin declines as people age, which makes deficiency more likely in the elderly. The consequences of vitamin B12 depletion -- anemia and potentially irreversible nerve damage -- prompt many people to take large doses of vitamin B12, often upon the advice of their physicians.

Dosage Limits

Your daily requirement for vitamin B12 is measured in micrograms, rather than milligrams. According to the Office of Dietary Supplements, daily requirements for vitamin B12 range from .4 mcg for infants to 2.8 mcg for lactating females. No upper limit has been established for vitamin B12, as it is nontoxic even in megadoses, according to Dr. Elson Haas, author of "Staying Healthy with Nutrition." Vitamin B12 is frequently taken in doses one thousand times the daily requirement without ill effects. However, injections of synthetic vitamin B12 have been associated with side effects, including flushing, dizziness, diarrhea, rash, itching and even anaphylaxis.

Considerations

Because vitamin B12 is stored in your liver for a long time, deficiency may take many months or even years to develop once absorption or dietary intake of the vitamin falls below your physiologic needs. However, because vitamin B12 deficiency is associated with serious consequences, many physicians recommend prophylactic B12 supplementation among certain groups, such as the elderly or vegetarians. There is no official tolerable upper limit on vitamin B12 dosage, and oral doses of 1,000 to 2,000 mcg daily are well-tolerated.