Lysine is an amino acid (building block of protein). People use it to make medicine. Lysine is used for preventing and treating cold sores . It is taken by mouth or applied directly to the skin for this use. Lysine is an important additive to animal feed because it is a limiting amino acid when optimizing the growth of certain animals such as pigs and chickens for the production of meat. Methionine and lysine are essential amino acids that are critical for the growth of body tissues2,3. Bronstein and Lepstein4 reported that feeds containing 19.1% crude protein, when supplemented with methionine, would produce similar growth and efficiency as feeds containing 23% crude protein.