Activyl Spot-On for Dogs is the first topical flea treatment to contain indoxacarb, an innocative active ingredient that activates using enzymes inside the flea. Once in contact with the parasites, Activyl's full flea-killing power is activated to eliminate all adult fleas and break the flea life cycle once and for all. The waterproof, fragrance-free formula keeps your kitty protected and itch-free for a full 4 weeks. Features innovative active ingredients and mode of action. Fast-acting. Starts killing fleas within 8 hours. Breaks the flea life cycle to prevent reinfestation Quick-drying and fragrance-free. Waterproof, remains effective after shampooing and bathing. Activyl is a registered trademark of Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co. Inc.