Description – Lysine is an amino acid (a building block of protein). Unlike some other amino acids, the human body cannot make lysine, so it is supplemented through the diet. Sources of lysine include meat, fish, dairy, eggs, and some plants such as soy. Function – The most common use of L-lysine HCL in dietary supplementation is to aid with the natural recovery of sores. Lysine is taken by mouth or applied directly to the skin for preventing and treating cold sores (caused by the virus called herpes simplex labialis). Lysine is taken by mouth to improve athletic performance and for improving symptoms of schizophrenia.