Advantage XD Large Cat offers lasting protection against fleas packing two months of continuous treatment into just one dose. This topical treatment starts killing fleas through contact in 30 minutes to 4 hours eliminating fleas before they can bite your cat. With Advantage XD extended duration formula flea prevention remains effective for 8 weeks outlasting the typical 6-week flea life cycle and preventing reinfestation. For your convenience Advantage XD is available over the counter ? no prescription needed. Advantage XD Large Cat is effective on both indoor and outdoor cats at least 8 weeks of age and weighing over 9lbs.