Nioxin System 1 Leave-In Hair & Scalp Treatment for Lightly Thinning Natural Hair works to refresh the scalp and increase the volume of your hair. This volumizing leave-in treatment helps provide thicker-looking hair by increasing the fullness of each hair strand. Nioxin System 1 Hair & Scalp Treatment is the final step in the 3-part system, amplifying hair texture while protecting against breakage. How to use Apply treatment to damp hair and scalp. Use up to nine product pumps. Distribute the product evenly on the scalp, massage and comb. Do not rinse.