I had a kitten several years ago, she was about 5 or 6 months old and she had pretty bad fleas when I got her. I bought a Hartz flea collar and put it on her. The next day when I got home from work my neighbor was waiting outside for me to tell me my kitten was dead in the bushes. She had not been injured in any way, just laid down in the bushes and died. I always figured she had been poisoned although I could never figure out how that could have happened. I always wondered to myself if the collar had had anything to do with it but because she had been the proper age the label listed I didn’t think that should have caused any problem. Now I think that is exactly what it was. No proof, of course, but I can tell you I have never used a flea collar on another cat and never will.