Finding the best heartworm preventative medication for your dog is a decision that is best made in partnership with your veterinarian. The right heartworm preventive medication for your dog depends on a lot of factors you can discuss with your veterinarian. The great news is there are a lot of options, which include injectable long-lasting preventative, chewable, topical application – and combinations that also prevent against internal parasites like roundworms and external parasites like fleas and ticks. Depending on the lifestyle and needs of you and your pet, you can select from many options. Breed is also taken into consideration when choosing the best heartworm therapy for your pet. Some breeds such as collies, may have an increased sensitivity to the active ingredient Ivermectin.