Heartgard treats heartworm disease, a serious infection that can be fatal to your dog. This disease is spread to dogs through the bites of infected mosquitoes. Heartgard helps prevent heartworm disease by killing the worms before they grow into adults and multiply in the bloodstream. It comes in the form of a chewable tablet that's given once every month to dogs six weeks of age and older and is safe for dogs of all sizes regardless of weight, including pregnant or nursing females.