We do not use any heartworm medication on our dogs! That may sound crazy to you but did you know the monthly heartworm preventatives aren't actually a preventative at all?!? Here is an article from Dogs naturally magazine explaining the cycle of heartworms and exactly why we think it's best to skip the monthly meds. be sure to read all the linked articles within this one as well. You will have a much better understanding of the "disease" you aren't preventing.