Mary, When I took the four ferrets for the vet visit, she gave me "interceptor" heartworm tablets, and told me to give each ferret 1 pill a month, for 12 months, so here is a second professional opinion. Yes, as it seems they take it once a month for 12 months. The new ferrets are adjusting so well, Sugar, who came to us flighty and aloof, is becoming attached, by following me around the house. And she is getting along with the others. They haven't started giving kisses yet, but that day will come. Marti Loosewire, Lucy, Sugar & Spice [Posted in FML issue 1722]