Vitamins are required for life. A minimum daily dose of each vitamin is necessary to maintain good health. Significantly exceeding this dose, however, can cause illness. In general, the symptoms of vitamin toxicity include nausea, gastrointestinal problems like constipation and diarrhea, hair loss, rashes, and nerve damage.
It is unusual to overdose on vitamins through megadose supplements, but people sometimes receive an unsafe dose of some vitamins by combining fortified foods with supplements.
Vitamin overdose and toxicity rarely leads to death or serious illness. In 2012, the American Association of Poison Control Centers reported 59,028 exposures and only one death.
As B vitamins are often given as supplements and found in fortified foods, there is some risk of taking too much B vitamin. There are eight B vitamins; thiamine, ribovlavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid, and cobalamine. Each functions as an enzymatic cofactor or is a precursor to an enzymatic cofactor enabling many of the basic functions of metabolism in the body.
The recommended daily allowance (RDA) of vitamin B1 (thiamin) is 1.5 mg per day for an adult, and 0.7 mg for children age 1 to 4. Thiamine is generally nontoxic.
The RDA for vitamin B2 (riboflavin) is 1.7 mg for adults and 0.8 mg for children age 1 to 4. Vitamin B2 is also generally nontoxic.
The RDA for vitamin B3 (niacin) is 20 mg for adults, and 9 mg for children between 1 and 4. There is no toxic dose established in humans. However, at doses higher than 50 mg per day, some side effects such as skin flushing can occur. Therapeutic doses of 1500 to 1600 mg per day can be given, but with a risk of liver toxicity, especially in the presence of pre-existing liver disease. There were 1374 exposures to niacin toxicity reported in 2015.
The recommended adequate intake of vitamin B5 (pantothenic acid) is 5 mg per day for adults. It is not known to be toxic in humans and there is no tolerable upper intake level established. Diarrhea has been documented at intakes of 10 to 20 g per day.
The RDA for vitamin B6 (pyridoxine) is 1.3 mg for adults between 19 and 50 years. . An acute toxic dose has not been established but it is known that vitamin B6 may cause neurotoxicity at a dose of 300 to 500 mg per day over time. In 2015, 189 toxic exposures were reported for Vitamin B6.
Vitamin B7 (biotin) is a cofactor for key enzymes in the process of gene regulation. The recommended intake of biotin is 30 mcg per day in adults. Biotin is not considered to be toxic, and no tolerable upper intake level has been established.
The RDA for vitamin B9, folic acid, is 400 mcg per day for people over 14, 600 mcg for pregnant women, and 500 mcg for lactating women. Requirements for children are dependent on age. The safe upper limit of folic acid for adults is 1000 mcg from fortified foods and supplements. Taking more than that could conceal the signs of a vitamin B12 deficiency in older people.
The RDA for vitamin B12 (cobalamine) is 2.4 mcg for people over age 14. The RDA for children is dependent on age. No tolerable upper intake level for vitamin B12 is established.