We have just adopted the most gorgeous, friendly little chap. The cats protection people told us he has a stronghold allergy as he has a little bald patch on the back of his neck where they put the drops.
I will chat to the vet about it but I would be so grateful if anyone else has had a cat with this allergy could let me know what works for them. We are very rural and so there are loads of flea ridden beasties out all around so he does need to be protected when we eventually let him out!