http://www.petrxshop.com/blog/2009/04/10/my-vet-told-me-i-didnt-have-to-test-for-heartworms-if-i-gave-my-dog-the-heartworm-prevention-medacine-before-2/ Pet Medication and Supplies for your Pet hourly 1 http://wordpress.org/?v=3.1.3