One of my cats reacts to Revolution, but the other does just fine. A few days after application she develops a sore red spot, loses fur, and then gets a scab, which heals and eventually the fur grows back. The first time it happened it was just a very small sore, but she was on the small dose also. Then I kind of forgot about it and she got the stronger dose and a very sore area. The vet said that happens to some animals, her own dog reacts also, and to try it on 3 areas instead. It still happened with a big sore spot in the same area, and a very small sore in another. Obviously I won't use this on her again.
But like I said, the other cat has no reaction.
We will try something different, and I'm only going for a one month dose for a trial, or possibly a free trial, if offered.