L-cystine is an amino acid required by the human body to build protein structures. L-cystine has antioxidant properties and helps protect cells from damage caused by free radicals. For this reason, L-cystine is widely used in hair and skin care products to support healthy hair, reduce dark spots, brighten the skin, and help slow skin aging.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6017824/