My 15 year old Siamese has fleas. When I took him to the vet he said that he didn't have that many and gave us Frontline Gold for Cats. This is the 4th month and he is still pulling fur out. When I use the flea comb all I get is some flea dirt. He is a indoor cat and we vacuum every 3 to 4 days. I sprayed the carpet and area with a flea spray. My question is can I use Vet's Best topical spray while he is on Frontline?