The 2-pack of k9 advantix ii flea and tick prevention for extra-large dogs is a broad-spectrum, veterinarian-recommended dog flea and tick prevention that also kills and repels mosquitoes, a benefit not found in many other flea and tick products. some products require a flea or tick to bite your dog in order to work, but k9 advantix ii flea and tick prevention for dogs is a topical formula that works through contact, so your dog doesn't have to endure a painful bite for the flea, tick or mosquito to die. just one application of k9 advantix ii flea and tick prevention for extra-large dogs starts working to kill fleas within 12 hours, is waterproof after 24 hours, and continues to work to kill fleas, ticks and mosquitoes for up to 30 days, even after a bath or swimming. we recommend the use of a grooming shampoo designed specifically for pets. do not use k9 advantix ii on cats. use only on dogs.