Do you need the heartworm blood test if you give heartworm pill 12 months a year?

Yes, it is still recommended that dogs be tested yearly for heartworms even if they are on a heartworm preventative because it is possible for the dog to vomit up all or a portion of the pill without the owner knowing and thus allow a window for infection. The same is true of the topical heartworm preventatives, though the concern isn't vomiting but improper application.