It depends upon the product that was used to bath the dog. The Hartz shampoo should be in a different category of flea preventative than the Frontline so it should be safe to use at any time. There is another product that can be used in combination with Frontline--it is called Capstar and kills adult fleas within 24-28 hours, then Frontline is used for prevention. The problem may be that your dog's environment (house and yard) is infested with fleas. So even after treatment the environment is reinfecting your dog. You need to treat not just the dog but the environment for fleas.