Remember that chocolate and xylitol, both safe and delicious for humans, are toxic for dogs (among other things that are also toxic for dogs). So it's not unusual for something to have side effects in a dog but not having them in a human. Veterinary medications are typically priced at much lower levels than the same medication for humans -- which is why some people have been known to get prescriptions from a friendly vet, to use for themselves.

It seems logical that the first step is to get approval for humans, then it would be easy to get it for animals.