My roommate works at a vet and gives my ferrets 1/2 a dose of the kitten
Advantage to their necks.  So, one dose for a kitten is good for two
ferrets.  The advantage goes by their weight.  As, far as we know their is
not one designed for ferrets and we were a little leary of giving it to
ours, but they do fine.  Apparently it tastes very bad, so they don't lick
it off each other or anything.  It did help rid the flea problems we were
having when nothing else seemed to.  If you do try Advantage make sure to
wash all of their beddings etc.  There may be flea eggs in them.
[Posted in FML issue 2140]