Seresto large dog collars kill and repel fleas and ticks for 8 continuous months. Seresto collars make your dogs comfort a priority by killing fleas and ticks through contact, so pests don't have to bite your dog to die. Existing fleas die after 24 hours of initial application of the collar. After 24 hours, any new fleas on the dog are killed within 2 hours with protection against further flea infestation lasting 8 months. The prevention of infestations with new ticks starts at 48 hours after application of the collar. 48 hours after application, any new ticks are repelled and/or killed in as quickly as 6 hours. Protects against common pests like deer ticks, American dog ticks, brown dog ticks, and Lone Star ticks. They are also effective against chewing lice for 8 months. For large dog flea and tick collars are for dogs at least 7 weeks old and over 18 pounds, and no prescription is needed. They're odorless, non-greasy and water-resistant to swimming, exposure to rain or sunlight, bathing, and shampoo treatment. Water resistant and remains effective following a shampoo treatment, swimming or after exposure to rain or sunlight. Under normal conditions, effectiveness lasts for 8 months. In order to maintain an 8-month duration, dogs must not be bathed more than once per month. For dogs that swim once a month or more, the control duration is reduced to 5 months for flea control and reduced to 7 months for tick control.