The heartworm medicines are a scam. The educated way to deal with the heartworm threat is to test your dog for heartworms three times a year (i.e. once every 4 months).
If the test results continue to be negative, there is no reason to give the dog heartworm medication EVER.
If a test is positive, give your dog a regular dose of ivermectin, and that will take care of the larvae. Remember, 4 months is not enough for adult heartworms to develop, so the treatment will be the same as the so called "preventative."
More reading:
Are drug companies honest about heartworm?