ZoGuard Plus for Dogs contains Fipronil and S-Methoprene (insect growth regulator) to effectively kill fleas and ticks (including those that may transmit Lyme disease). The active ingredients in ZoGuard Plus for Dogs feature a sustained release formula developed for fast-acting and long-lasting control of fleas, ticks, and chewing lice for up to 1 month on dogs. Fipronil is the number one active ingredient available for control of fleas and ticks. S-Methoprene breaks the flea lifecycle by preventing flea eggs and larvae from maturing.