My boy has got the dreaded fleas. He has some bites/irritated skin around the head. If I can find some clear skin around the back of the neck is it still ok to use spot on? I know alot of these products say do not use on infected skin? But oral meds aren't as effective and the poor little bugger has a separate procedure (CT scan) booked for Thursday, so I could do with the fleas being wiped out before then...