We use Advantage II on the cats here and haven't had a problem with it. The only change we make with the dosing is that, although 2 of the 3 cats run somewhere around the 10 lb mark (our housemate's 2 are much bigger but he uses Frontline on them), we use the dose for small cats (up to 9 lb). Rosa, of course, has health issues so we want to minimize her dose and her twin, Regan, is allergic to Frontline so we try to minimize the risk of a reaction to the Advantage as well for her. Licorice is a smaller cat anyway so the small dose is the correct one for her. We've had no problem getting rid of any fleas that show up with the slightly reduced dose and we haven't seen any issues arising from using a 'chemical' treatment (I hesitate to use the term chemical because technically everything is a chemical, but it seems to be the term that many people understand best) on Rosa either.