Seresto collars are effective for killing fleas and ticks for up to 8 months when use on a monthly basis. An active infestation may take longer to kill. The Seresto collar will start working as soon as it is placed on your pet’s neck, but don’t expect immediate results as there is an initial period in which the fleas and ticks will succumb to its protection. The Seresto collar can last up to 8 months if it is worn by your pet without interruption. It should not be removed before 8 months unless advised otherwise by your veterinarian. During this time, the active ingredients in the collar, imidacloprid and flumethrin, repel and kill existing pests that come into contact with your pet. Your pet should stay protected throughout the duration of wearing the Seresto collar as long as all instructions are followed correctly.