"Heartguard" the most popular heartworm medication sold by vets is nothing but ivermectin and pyrantel, two very common worm and parasite killers. You can get them from Tractor Supply in paste forms meant to be used with livestock, like cattle and horses. The cost is only pennies per dose. Yet the same stuff packaged for your dog costs $10.00 per dose? If I was sure I'd get the dosage right, I'd tell the vet to go screw and mix my own heartworm medicine up in the kitchen.