We put Hartz on our cat because my girlfriend thought that it would be O.K. since she had applied K9 Advantage to her grandmothers cats in the past without incident. We love our cat dearly and this was an honest mistake that is commonly made. I just want to tell my story so it may help the next person. About an hour after applying the treatment my cat started acting weird and then I awoke to her crying in the middle of the night. She was in a lot of pain and I could see her twitching her ears and shaking her head as her nervous system had been poisoned by the permethrin. This condition is known as permethrin toxicity and there is no antidote for it. The best thing to do is to wash your animal with Dawn dishsoap because Hartz is an oil based solution and the Dawn does well to break it down. After washing my cat with the Dawn I saw an almost immediate improvement. The muscle spasms stopped within 36 hours. We are very lucky to still be graced with our cats presence. These over the counter medicines are really not worth the trouble and you should consult your vet before using any of them. Most of these flea spot treatments are manufactured in China and other places in the world with a cheap labor pool. The government is in the process of investigating these products as incidents are on the rise. See article.
I hope our experience can help someone else.