Frontline Plus for Dogs has 2 tough killing ingredients. The 1st, fipronil, kills adult fleas and ticks. The 2nd, (S)-methoprene, attacks flea eggs and larvae. They work together to kill all month long. FrontlinePlus gives you the power to help prevent a flea infestation on your pet. Its long-lasting formula is stored in the oil glands of the pet's skin to give non-stop flea and tick protection for a full 30 days. In a recent study, it was shown that the powerful ingredients of FRONTLINE Plus work to kill adult fleas within 12 hours of application. They then go on to break the flea life cycle by destroying the next generation of fleas. Its long-lasting formula is stored in the oil glands of the pet's skin to give non-stop flea and tick protection for a full30 days.