Pro Longer multi-benefit leave in treatment for long hair helps to thicken the appearance of your long hair whilst making it visibly healthier and stronger looking. When used with the Pro Longer shampoo for long hair, there is a hair breakage reduction *. The Filler A-100 technology is patented to L\'Oreal Professionnel, and it works by penetrating the fiber core to plump and thicken the hair from the inside. The formula works on thin ends, or areas where help is most needed. *instrumental test, after application of Shampoo leave-on