Heartgard

[kb7722]

Proheart and Advantage Multi do not use ivermectin as the active ingredient and they seem to be working best at this time. Heartworms have built up resistance to ivermectin. Those giving it orally are giving higher doses than that used in Heartgaurd. Ivermectin is not labeled for oral use on dogs. It will work in some breeds, but it is known to be toxic to certain breeds such as collies and other herding breeds. So there is a reason your vet is recommending products labeled for heartworm treatment in dogs and not because he is trying to get rich.

http://www.awca.net/drug.htm