It's commonly known that Hartz Mountain products are garbage. My mother taught me that when I was very little. We always used Advantage for flea control and it works great. It even is safe to use on rabbits. Of course the argument can be made that we don't yet know what the long-term effects of imidacloprid are on animals, but so far, so good.
The very best defense against parasites and diseases, however, is having indoor-only pets.