I actually make this post about every 3-4 months, since there always seem to be new people who don't know. We've dispensed literally thousands of doses of Advantage for ferrets. The worst side effect is occasional irritation of the skin where it's applied. You can split a small cat dose between two ferrets, or a larger dose among multiple ferrets. Advantage is all the same strength, and all you need is 0.2 ml per ferret. It works. It's safe. It's easy. It's much safer than all the sprays, powders, carpet treatments and most of the herbal treatments. Do yourself and your ferrets a favor by trying it. Dr. K. [Posted in FML issue 2476]