I've been using small cat/kitten Advantage on my ferrets as needed (in other words only when there is a presence of fleas) for probably about ten years now, and have never had any health issues as a result. I used it two months ago on my ferret who is adrenal, insulinomic, and lymphomic [how's that for a ferret health home run? :( ] and he tolerated it just fine. One or two drops is all it takes--one tube is enough for five ferrets! Anne [Posted in FML 6010]