Alfie was the same with Frontline.....
he runs around like mad trying to get it off and then he seems to stare into space and is just not himself.
I took him to the vet but he said it does not go into their system and he had never heard of anyone else having this type of reaction!!!
I dont know what to do either as I worry if it does build up in their sytem although the vet said that was impossible!!!