Nioxin’s System 4 Leave-In Scalp & Hair Treatment for Colour-Treated Hair with Progressed Thinning works to refresh the scalp and increase your hair volume. This volumizing leave-in hair treatment helps to provide thicker-looking hair by increasing the fullness of each hair strand. Nioxin’s System 4 Scalp & Hair Treatment is the final step of the three-part system, which amplifies hair texture while protecting against breakage.