Lufenuron does not kill adult fleas, it breaks the life cycle of fleas by adult fleas ingesting the chemical then adult fleas transfer it to their eggs. This is from peteducation.com:
FDA approved for use in dogs and cats. Available by prescription and over the counter. Once the female flea ingests blood from a pet treated with lufenuron, 96% of egg development from fleas on cats and 99% of egg development from fleas on dogs is stopped. This helps prevent a continual flea problem. Lufenuron does not kill the adult flea and does not stop the flea from biting and causing flea allergy dermatitis. The drug is stored in the body fat and released into the bloodstream over the course of a month. Flea eggs laid prior to treating the pet may take several months to hatch; Program will not be effective until these fleas start to lay eggs. Therefore it may take several months to see the product’s effectiveness. If quicker results are needed, use a product which will kill adult fleas; these will provide quicker relief for the pet. Cats require a higher dose per pound than dogs. After the cat injectable form is administered, 2-3 weeks are needed to reach therapeutic levels in the blood. The injectable form for cats is effective for 6 months.
http://www.peteducation.com/article.cfm?c=26+1303&aid=1471
