L-cysteine is a protein amino acid that exists naturally as a protein in most living organisms. Although most cysteine is found in proteins, small amounts of cysteine are also located in body fluids and in plants in non-protein form. The average diet of most individuals contributes approximately 1 gram of L-cysteine daily.

L-cysteine is considered a nonessential amino acid, meaning that sufficient amounts are produced by the body itself. Cysteine is one of the few amino acids containing sulfur. This allows cysteine to bond in a special way and to maintain the structure of proteins in the body.

L-cysteine aids in the synthesis of proteins, glutathione, taurine, coenzyme A, and inorganic sulfate in the body. Glutathionine itself has a number of biochemical functions, including maintenance of normal cellular redox state. Certain conditions, e.g. an acetaminophen overdose, can deplete hepatic glutathione, and this can be life-threatening. The antidote to an acetaminophen overdose is L-cysteine in the delivery form of N-acetylcysteine. The L-cysteine derived from N-acetylcysteine helps to restore hepatic glutathione.