My mom is convinced that her dogs have become immune to the Frontline Plus she's been using for the past several years. I don't see how this is possible, sigh, but what can you do?

Can you guys tell me what YOU use on your dogs? She wants a topical one, like Frontline, the liquid you just put on them. Oh, and we've tried the Hartz brand, and it sucks!! She wants the good $$$ stuff.

She thanks you for your help, in advance!