I use a combined product, Combinex, which is levamisole and triclabendazole, at this time of year, a pure flukicide Nov/Dec and a combined product pre-lambing, since I also want to worm them now and just before lambing.
Triclabendazole is the same stuff as in Fascinex so should be just as effective, but yes, you don't want to be using wormer when you don't need it, hence using pure Fascinex mid-winter.