I purchased the stuff from a reputable large pet store chain, thinking it will deter summer fleas and mosquitoes (since I have read about heart worms I was worried). I put it on my cat at about 8 pm and noticed the strong, oily greasy stuff and thought it would subside by morning. I woke up that morning and to my surprise my kitty was not waiting for me. He is always there when I wake up and walks me down stairs to start the day. I called for him and started searching after five minutes I started to panic. I finally found him hiding under the couch, very usual for him. I examined him and notice how lethargic he was, panicking further, I read the package and it said nothing of this. I washed him off hoping that would help, it didn?t. I rushed him to emergency vet services and they said it was the worse product to use on my beloved cat and this is a very common reaction to this product. For the first time he hissed at me, the vet said he is not himself due to this product. The clincher, as I was leaving the emergency hospital another cat was being admitted for the same problem. I took him to his doctor and he spent all day having tests ran and shots to counter act this reaction, $400 dollars later he is doing better. My cats vet and 2 emergency services confirmed that this product is not effective and very harsh on a cats body due to the ingredients used. The vet also stated Hartz is not regulated as Frontline is. Needless to say I will never use their products again.