Heartgard is the brand name of the anti-parasitic medication ivermectin made by Merial. There is also a version called Heartgard Plus, which is a combination of ivermectin and pyrantel. These chewable tablets are given as a preventative to heartworms. According to Merial, they should be given once a month every month, but only if the dog is free of adult heartworms.
Heartgard does not kill adult heartworms. Before starting a dog on Heartgard, the dog needs to have a heartworm test. Only melosomine dihydrochloride kills adults.
According to "Dog Owner's Home Veterinary Handbook," heartworm larvae that are in their second and third moults (called the L3 and L4 stages) can be killed by Heartgard. During this time, they live in the dog's bloodstream and not the heart, where Heartgard cannot reach.
Give one tablet as directed once per month. More than that and the dog will get ill. Because of the meaty flavor, dogs eat it voluntarily.
If a dog has been successfully treated for adult heartworms, "Dog Owner's Home Veterinary Handbook" points out they should not be given Heartguard until 4 or 5 weeks later. This gives the dog's bodies a chance to recover from the strong melosomine dihydrochloride.
Dogs can become infected with heartworms over the winter, so Heartgard should be given every month.