In addition, Frontline breaks the parasite life cycle, thus preventing them from laying eggs and reproducing. Frontline is applied to the back of the neck on cats and between the shoulder blades on dogs. It spreads naturally on the skin surface at the hair root level, aided by body movement to provide whole body coverage. Frontline is water-resistant flea medication, so it continues to kill fleas even after a shampooing, swimming, rain or exposure to sunlight. Frontline is for dogs and cats over 10 weeks old.