I have no personal experience with this product and my knowledge is based solely on an internet search. Proponents say it will prevent heartworms from forming. However, most advocate mosquito control as part of the therapy. If the dog is never bitten by a mosquito, it will never get heartworms. Being in the desert, we do not use heartworm preventative on a monthly basis at the sanctuary. I recommend to my family in Oklahoma the product Heargard Plus.