My vet gave me Frontline to use on my ferrets, in addition to my cats and
dogs.  He's teaches a Ferret class at the vet school at Virginia Tech,
has owned ferrets himself for 26 years, and even keeps two as "office"
pets who occasionally act as blood donors.
 
He said to use the new spray version, two sprays per pound on the
ferrets, one spray per pound on the cats.  He says it's much safer than
anything being sold over the counter.  The primary ingredient is used
for hair lice on children in Europe, and my vet said he wouldn't hesitate
to use it on his children if they were to have lice.
 
So, that's one vet's opinion, but one who's a bit of an expert.
[Posted in FML issue 3941]