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If you are friends with, or can make friends with, either a vet or vet tech, here's a little insider secret: heartworm treatment comes in single-use vials, but is drawn up by weight. The rest is (supposedly) discarded. If your vet is treating a small toy dog for heartworm, he might be willing to split the vial with your dog, saving you a lot of money.

My wife is a groomer who used to work at a vet's office. Our (formerly heartworm positive) dog got treated for cheap.