The effectiveness of Heartgard Plus does not vary based on the dog's age. Whether a dog is a puppy or an older adult, Heartgard Plus works similarly to prevent heartworm disease. However, Heartgard Plus for puppies can be administered as young as 6 weeks old, according to the manufacturer's label. The American Heartworm Society advises, however, that heartworm testing be done before starting prevention and that puppies be started on heartworm prevention as early as 6–8 weeks of age.