We put a Hartz flea collar on our 8 year old Siamese cat. He went from a very ative cat to a almost comatose, vomiting cat. His health previous to this was excellent. His appetite was also excellent. After this behavior continued a couple of days, we were trying to pin down what happened to him in the past 48 hours, and the only thing was the flea collar. Thank god we took it off, and by the next day he was back to his normal self!