It's very dangerous. I had a cat we almost lost due to hartz flea control "crap". She had a severe allergic reaction to it and we found her barely breathing with a think crusty foam forming around her nostrils and mouth. We called an emergency vet and they told us to get her into a warm bath and get all the flea product off of her and keep her in a warm steamy bathroom. To stay with her and if we did not see an inprovement within 30 minutes to call ( she gave us her home number) and to bring her in immediately. Luckily the warm bath to remove all the residue of the flea product and the streamy room cleared her up. She went the next day to the vet and that is when we were told about the product being unsafe and that it could have been much worse. Our vet at that time told us that she had seen several cats pass on due to the effects of the stuff.