I have used Frontline for 14 years (my oldest dog is 15) and never had a problem.  I lived in Florida for 7 years and my dog never had fleas.  When my oldest dog was very young another dog in the house had fleas but we considered that dog too old to apply a pesticide (Frontline is a pesticide) but we treated my dog, and since they both slept in the same bed, both dogs were rid of fleas!  When I worked at PetSmart, I cannot tell you how much of the over the counter flea stuff was returned because it didn't work.  Yes, Frontline is expensive, especially if you are treating multiple animals, but...