Lufenuron is stored in the animal's body fat and transferred to adult fleas through the host's blood when they feed. Adult fleas transfer it to their growing eggs through their blood, and to hatched larvae feeding on her excrement. It does not kill adult fleas.
Lufenuron, a benzoylurea pesticide, inhibits the production of chitin in insects. Without chitin, a larval flea will never develop a hard outer shell (exoskeleton). With its inner organs exposed to air, the insect dies from dehydration soon after hatching or molting (shedding its old, smaller shell).
Lufenuron is also used to fight fungal infections, since fungus cell walls are about ⅓ chitin .
It has no toxic effects at any dosage on humans, other mammals or any animals not having hard shells (like shellfish and insects have).