The Seresto Large Dog Vet-Recommended Flea & Tick Treatment & Prevention Collar for Dogs Over 18 lbs. is fast-acting and long-lasting to protect your furbaby from pesky pests. A paw-fect choice for cats and kittens, this collar starts to kill existing fleas on dogs quickly, and then it continues to work for eight months. The collar is non-greasy and odorless. Since it kills fleas on contact, they don’t have to bite your dog to die, so the collar helps to keep your furbaby comfy.