I think it worked. I used it once when we lived in Oklahoma for 6 months. I'm from Reno and in Nevada it's difficult for fleas to survive so rarely do dogs need this medicine. As a matter of fact, I've owned dogs my whole life and NEVER used any flea meds until we lived in a humid hot area, Oklahoma. So I think it worked since my dog didn't get fleas but because I only used it once I can't give a huge explanation of my rating, and I can't give a long term rating, it was a one time thing.