My dog Boris got chemical burns from Frontline. He had several circular bald patches down his back in the exact spots I put the Frontline. Hair never grew back in those spots. Around the same time several years ago, after applying Frontline, Bonni ran around the house rubbing her back on whatever she could, and then she ran into the shower and stood there. I washed it off. So after those experiences, I never went back to Frontline.
Last year I tried Flea Free (vinegar,garlic, and herbal stuff added to their food), and Liquid Fence (herbal spray). Seems to work, at least I don't think they can hurt.