l-cysteine

L-cysteine is a sulfur-containing amino acid classified as "semi-essential" or "conditionally essential," meaning the body can synthesize it from methionine and serine under normal conditions but may require supplementation in certain situations (e.g., infants or individuals with metabolic disorders)[1][3][5]. It serves as a precursor for the synthesis of glutathione—one of the body's most important intracellular antioxidants—and taurine[2][3][6]. L-cysteine is used therapeutically as an additive in parenteral nutrition to meet amino acid requirements in newborns (including preterm infants) and patients with severe liver disease who cannot adequately synthesize it[4][5]. It also has antioxidant properties and plays roles in protein synthesis, detoxification processes (notably via glutathione), immune function support, respiratory health (as a mucolytic), and maintenance of skin/hair/nail health[1][6]. While sometimes promoted for uses such as cancer prevention or diabetes management, robust clinical evidence supporting these indications is lacking[1].

Brand names

CystechElcysNouress

Other names

cysteineL-cysteine hydrochloridecysteine HClcysteine hydrochloride monohydratea-amino-b-thiolpropionic acid