The menadione compounds allowed are menadione dimethylpyrimidinol bisulfite (MDPB), menadione nicotinamide bisulfite (MNBS) and menadione sodium bisulfite complex (MSBC). As for the toxicity concerns, there are no reports of nutritional toxicity of any vitamin K sources in dogs or cats readily available in the literature, and certainly no descriptions that could be found showing vitamin K3 (menadione) was harmful to dogs, cats or other domestic animals. -- Nutrient Requirements of Dogs and Cats (National Research Council, 2006)

University of North Carolina
Menadione Bisulfite or Vitamin K is a hepatotoxicant that damages periportal regions of the lobule in perfused liver in an oxygen-dependent manner. The effect of ethanol on menadione bisulfite toxicity was examined in perfused rat liver. 
-- PE Ganey, Y Takei, FC Kauffman and RG Thurman, Department of Pharmacology, University of North Carolina, Chapel Hill.

From Blue Buffalo
Due to the extrapolation of the human research data about the toxicity of Vitamin K as a cancer drug, some consumers still worried about the potential harm to their pets, even though it has never been documented. BLUE elected to eliminate any worry from our loyal pet parents and removed Mendadione from our formulas as of October 2006. Because of the holistic nature of our diets, we felt that the inclusion of while some vegetables, and the normal bacterial activity in the colon, was enough to supply the required vitamin K needs of the dog and cats. Menadione had some unique metabolic activity besides blood clotting. Human researcher experimented with Menadione at high doses as an injectable treatment for some forms of cancer. Unfortunately, the side effects caused liver problems in some patients. So the FDA banned the use of Menadione in any form in humans. However, for the veterinary branch of the FDA (CVM) determined that the very low doses, required for vitamin supplementation orally, presented no risk to animals. As we speak, the use of Menadione as a vitamin K supplement is approved by both the FDA and AAFCO for animal use. - Blue Buffalo Company

Sabine Contreras
Also shown as dimethylprimidinol sulfate. The FDA has banned synthetic vitamin K from over-the-counter supplements because of its high toxicity. Vitamins K1 and K2 are metabolized through the lymphatic system, utilizing pancreatic enzymes and bile acids and regulated by the liver. Vitamin K3 is absorbed directly and bypasses the natural pathways and regulators. - Sabine Contreras, Animal Nutrutionist.