A large body of evidence has accumulated that treatment with riboflavin in excess of nutritional requirements has very little toxicity either for experimental animals or for humans (Rivlin, 1978). There are no reports of riboflavin toxicity studies in cats. Most data from rats suggest that dietary levels between 10 and 20 times the requirement (possibly 100 times) can be tolerated safely (NRC, 1987).
Dogs are particularly tolerant to large doses of riboflavin. Dogs given a single oral dose of 2 g riboflavin per kg (0.91 g per lb) body weight showed no ill effects (Unna and Greslin, 1942). Similarly, four 10-week-old puppies were fed 25 mg riboflavin per kg (11.36 mg per lb) of body weight for five months; neither toxic signs nor pathological changes in the organs at necropsy were observed. It would appear that feeding levels 42,000 times the NRC requirements for dogs caused no adverse effects (Unna and Greslin, 1942).
When massive amounts of riboflavin are administered orally, only a small fraction of the dose is absorbed, the remainder being excreted in the feces. Lack of toxicity is probably due to the fact that the transport system necessary for the absorption of riboflavin across the gastrointestinal mucosa becomes saturated, limiting riboflavin absorption (Christensen, 1973). Also, capacity of the tissues to store riboflavin and its coenzyme derivatives appears to be limited when excessive amounts are administered.
