I have two cats (both over 9 pounds) that have had fleas for a month. I bought them a spot on flea treamtent at Petsmart that did not work. It has been 3 weeks since I treated them. Would it be safe for me to treat them with Advantage now? I dont want them to overdose but all three of us are miserable...please help! Plus any helpful hints for fleas would be appreciated.