Cysteine hydrochloride is a water-soluble, stable derivative of the naturally occurring amino acid cysteine, which is a key component of hair keratin and various proteins in the human body. It is widely used in the personal care and cosmetics industry, especially in hair care products such as perm lotions, hair relaxers, hair repair treatments and shampoos.

As a mild reducing agent, its core efficacy lies in breaking and restructuring the disulfide bonds in hair keratin. Compared with traditional thioglycolic acid and its salts, it has a much milder effect, which can effectively reduce hair damage, dryness and breakage caused by the perming process. It is particularly suitable for formulating gentle perm products for damaged hair, colored hair and sensitive scalps. In addition to perming applications, it also has a strong hair repair function. It can penetrate the hair cuticle, supplement the cysteine content in hair keratin, repair broken disulfide bonds, enhance hair strength and elasticity, improve hair texture, and make dry and frizzy hair soft and smooth. It is also used as an antioxidant and a nutrient supplement in some skin care products and dietary supplements to help protect cells from oxidative damage.