Recently I used to hartz flea drops on my cats. I noticed they were trying to get it off of themselves as much as possible, and the next day the flea dirt (digested blood bits) seemed worse. I washed the cats who were reacting the worst, and went to the vet and bought the Advantage flea control. When I went to apply this to the younger of my two males, I noticed that he had blisters and his skin was peeling off where I had applied the Hartz product. I think this company should be put out of business!