Advantus is flavored soft chew formula that kills adult fleas fast - it starts working within 1 hour and achieves 96% effectiveness within 4 hours. A single dose kills existing adult fleas, but if your pet should become re-infested, Advantus is safe enough to administer as often as once per day. Not a preventive; works only on the adult stage of the flea life cycle. For dogs and puppies 4 lbs of body weight or greater and 10 weeks of age or older.