I talked to Pfizer today. They recommend the 5-15 lb cat size at this time. When they did some small studies with ferrets, 1 ferret did get heartworms with the 0-5 lb kitten size. A small safety study was done with the 5-15 lb size, and it was safe at this high dose. They thought there will be more info on ferrets at the Heartworm Society Meeting in April. Jerry Murray, DVM [Posted in FML issue 3371]