No, ProHeart6 is not approved for ferrets, but none of the heartworm
meds are actually approved for ferrets.  However most have been tested
in small studies to determine if they work in ferrets.  ProHeart6 has
not been studied yet in ferrets.  Perhaps some of the folks in Australia
where ProHeart has been out for a longer time may have some experience
with it in ferrets.
 
In the mean time go with Interceptor, Heartgard, ivomec, or Revolution.
 
Hope that helps,
Jerry Murray, DVM
[Posted in FML issue 3756]