Lysine is an amino acid (building block of protein). People use it to make medicine. Lysine is used for preventing and treating cold sores (caused by the virus called herpes simplex labialis). It is taken by mouth or applied directly to the skin for this use.Lysine is also taken by mouth to improve athletic performance.eople use it to make medicine. Lysine is used for preventing and treating cold sores (caused by the virus called herpes simplex labialis). It is taken by mouth or applied directly to the skin for this use. Lysine is also taken by mouth to improve athletic performance.