Toxicity
Riboflavin is not toxic when taken orally, as its low solubility keeps it from being absorbed in dangerous amounts within the digestive tract. Although toxic doses can be administered by injection, any excess at nutritionally relevant doses is excreted in the urine, imparting a bright yellow color when in large quantities. In humans, there is no evidence for riboflavin toxicity produced by excessive intakes. Even when 400 mg/d of riboflavin was given orally to subjects in one study for three months to investigate the efficacy of riboflavin in the prevention of migraine headache, no short-term side effects were reported.
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