B Vitamin Side Effects
Because water-soluble B vitamins are eliminated in the urine, taking large doses of certain B vitamins may produce transient effects.
| Vitamin | Name | Tolerable Upper Intake Level | Harmful effects |
| Vitamin B1 | thiamine | None | No known toxicity from oral intake. There are some reports of anaphylaxis caused by high-dose thiamine injections into the vein or muscle. However, the doses were greater than the quantity humans can physically absorb from oral intake. |
| Vitamin B2 | riboflavin | None. | No evidence of toxicity based on limited human and animal studies. The only evidence of adverse effects associated with riboflavin comes from in vitro studies showing the production of reactive oxygen species (free radicals) when riboflavin was exposed to intense visible and UV light. |
| Vitamin B3 | niacin | 35 mg/day from supplements, drugs or fortified food | Intake of 3000 mg/day of nicotinamide and 1500 mg/day of nicotinic acid are associated with nausea, vomiting, and signs and symptoms of liver toxicity. Other effects may include glucose intolerance, and (reversible) ocular effects. Additionally, the nicotinic acid form may cause vasodilatory effects, also known as flushing, including redness of the skin, often accompanied by an itching, tingling, or mild burning sensation, which is also often accompanied by pruritus, headaches, and increased intracranial blood flow, and occasionally accompanied by pain. Medical practitioners prescribe recommended doses up to 2000 mg. per day of niacin, usually in time release format, to combat arterial plaque development in cases of high lipid levels. |
| Vitamin B5 | pantothenic acid | None | No known toxicity |
| Vitamin B6 | pyridoxine | 100 mg/day from supplements, drugs or fortified food | Intake of more than 1000 mg/day is associated with peripheral sensory neuropathy; other effects are unconfirmed: dermatological lesions ; B6 dependency in newborns . |
| Vitamin B7 | biotin | None | No known toxicity |
| Vitamin B9 | folic acid | 1 mg/day | Masks B12 deficiency, which can lead to permanent neurological damage |
| Vitamin B12 | cyanocobalamin | None established. | Acne-like rash . |
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