Fleas seem to be resistant to the products I've tried in the past, so I tried a new product by Merck called Activyl and it worked amazingly well! Activyl is a topical flea treatment you apply down your dogs back. It’s the first that contains the ingredient INDOXACARB. Indoxacarb works by reacting with enzymes inside the flea. Fleas don’t have to bite your dog – they absorb Activyl by coming into contact with the dog’s skin.