Vitamin K3: is it unnecessary and toxic?

Vitamin K is one of those nutrients that we learn of early in nutrition training as an essential fat soluble vitamin that is important in clotting. Beyond that, we seldom discuss it. Recently, however, there has been a concerted effort by a few "pet enthusiasts" to spook petfood manufacturers and well-meaning pet owners into the notion that vitamin K supplementation in the diet, specifically with vitamin K3 (menadione), is unnecessary and potentially toxic to pets. Determining whether they have a valid point warrants a bit of investigation.