I had actually used the Hartz crap on my cats a couple of years ago and it made them all sick. Usually very playful cats, they were mopey and sleeping all day long. I had used it once before and remember them having a similar reaction, but this time, it seemed worse. I went to the vet a couple of days later for their annual shots and told the vet about their listlessness after applying flea medicine. The vet asked if I used Hartz and I told them yes. The vet recommended that I wash it off of them immediately and use Frontline instead. I thought the vet was just trying to sell me his stuff, but I did as he suggested anyway just in case. Now, I’m SO glad I listened. My kitties are all OK and don’t seem to have sustained any of the horrible conditions that many others’ kitties seem to have.