2 different reactions

have two cats nearly 2yrs old. have used this product before, no problems. neighbour's cat has fleas and one of mine was scratching so used advocate on both cats. the long haired cat had no problems but the short haired one immediately had red skin and was trying to lick/scratch the site. as was between shoulders she couldn't get to it but she was not herself for 2 days but now she's back to normal.luckily I didn't need to take her to the vet but I won't be using this product on her again.
I would have liked the vet to have told me there could be side effects before selling me product, even though it had been used previously on the cats. Maybe short haired cats shouldn't use this product.