What is flea drop, is that a make or are you referring to spot on Frontline or other vet spot on.
If its a named spot on , I believe it will make them salviate a lot and obviously wont work very well if its all licked off. I am sure there will be instructions on the packet about it being licked and if you are worried give your vet a ring.
If its an over the counter, not Frontline, then I would be slightly more concerned as these dont work and arte more dangerous and would ring your vet.