Dr. Jean Dodds recommends the monthly PLAIN heartworm medicine given every 45 days. Basically the medications that just contain Ivermectin (although note that Ivermectin can be a problem for Border Collies and related breeds, so ask your vet for an alternative). Do not use the 6-month shots, use the monthly preventative so you have control in the event your animal experiences a reaction to the medication. It is an accepted practice among dog owners who practice holistic care that Interceptor and Heartgard are given every six weeks instead of every four because they are effective for that long. If you live in a state that does not have mosquitoes year round, it is also important to take your dog off the pills for a few months. In order for microfilariae to develop into the infective stage, it needs to be sufficiently warm (above 57 degrees) for a period of time. It takes approximately 6 months for the microfilariae to mature into adult heartworms, so stopping the drug for a few months will still allow for kill of the various stages when started up again. In fact, ask your vet -- if the adult heartworm load is not too heavy, it is possible to treat the dog using the heartworm preventative rather than putting them through the toxic treatment of arsenic to kill the worms. It is a good idea to give Milk Thistle or an herbal liver detox formula following a dose of the heartworm drug