The recommended dietary allowance, or RDA, for each of the eight B vitamins represents the amount needed to meet the nutritional requirements for 97 to 98 percent of individuals in the specified age group. The risk for overdose symptoms is greater for certain B vitamins than for others. If you take in more than the recommended dosage of a multivitamin, MedlinePlus advises calling the National Poison Control Center at 800-222-1222. For your safety, contact poison control or your doctor if you're concerned about a high intake of any combination of B vitamins as well.
Still Under Study
A few of the B vitamins are not associated with any adverse side effects at high dosages, although caution is advised due to the lack of data, according to the Institute of Medicine. Among those with no known problems and no established tolerable upper intake levels are thiamine, riboflavin, pantothenic acid, biotin and vitamin B-12. A well-balanced diet should provide adequate amounts of these B vitamins. If you use dietary supplements, look for brands that provide no more than 100 percent of the recommended daily intake.
Niacin Overdose
The Institute of Medicine cautions that excessive consumption of niacin, or B-3, from supplements might cause flushing and gastrointestinal problems. The American Cancer Society adds that high doses of niacin might worsen stomach ulcers or cause nausea or blurred vision, although such reactions are rare. The tolerable upper intake level -- which represents the highest intake of a nutrient you can take in without risking adverse reactions -- for niacin is 35 mg for adults.
Vitamin B-6 and Folic Acid
High intake of B-6 from supplements can cause nerve damage in the arms and legs, according to the Office of Dietary Supplements, although the damage might be reversed when supplementation is stopped. The tolerable upper intake for B-6 is 100 mg daily. Folic acid in high dosages might mask a vitamin B-12 deficiency. In high dosages, folic acid might also interfere with chemotherapy medications, according to the American Cancer Society.
Considerations
Although B vitamins are water soluble, and your body typically excretes any excesses through urine, you should consult your doctor about the right type and dosage of dietary supplements based on your individual needs. The American Cancer Society cautions that high dosages of B vitamins can affect your skin health and blood sugar levels, as well as cause gout. The best source of necessary vitamins and minerals is a well-balanced diet.
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