Thanks for using JustAnswer.com, I'm Dr. Dan and I'll try to help you.
What is the name of the flea medicine?
When where the 1st and 2nd dose applied?
Thanks for the information.
Usually nothing will happen. When the drug was released it was tested by putting up to 5x the dose on cats and none of them had any problems. If your cat licks the drug it can cause significant drooling but nothing else. If your cat has an allergic reaction then it will begin staggering and will have tremors. This is an emergency and a vet is needed immediately. However this is rare and likely nothing will happen to your cat except you wasted a little of the drug.
I hope this helps, best wishes.