As most people are aware, heartworm preventatives are important because mosquitoes biting your dog can cause heartworms. Heartworms are deadly if not treated and it's not the easiest treatment, for your dog or your wallet. It can be quite expensive to treat so it's also much cheaper to give your dog heartworm preventatives. If you have a dog, what heartworm preventative do you use? Which are the least expensive heartworm preventatives? Are you able to get a better deal somewhere online than at your vets?