I recently purchased Hartz over the counter. I thought I was getting a great deal since it was a 3-pack for $5 and I have three cats. I was wrong. The same day one of my cats started acting strange, secluding himself, and having trouble with his eyes I took him to the vet. The vet prescribed drops after I told her I had given him the flea medicine. She said I was LUCKY he only got an eye infection and that many animals have worse reactions. I think it was because although I put it between his ears on his head he was able to lick the access that spread and somehow got an eye infection that way. When I took him home I fed all my cats lunch and the same cat with the eye infection threw up his food immediately after finishing eating. I was worried because he has never thrown up food. Only hair balls. I searched online and found this website and decided to bath all my cats to get any medicine that was still on the surface of their skin and fur off. I will never use this product again and would advise others not to as well.