The first active ingredient is imidacloprid, which acts on the nervous system of the flea to cause paralysis and subsequent death. Within 12 hours of application on cats and dogs, 98-100 percent of fleas are dead. Advantage II Small Cat kills the existing fleas on ferrets within 24 hours. The second active ingredient, pyriproxyfen, is an insect growth regulator. Once a female flea lays her eggs, pyriproxyfen works to prevent the eggs from hatching, ending the flea life cycle. Also, the active ingredient in Advantage II kills through contact, which means that fleas don't have to bite your pet to die. Reinfesting fleas are killed with protection against further flea infestation lasting for up to 4 weeks. Pre-existing pupae in the environment may continue to emerge for 6 weeks or longer depending upon the climactic conditions.