NOW Foods L-Lysine (L-Lysine Monohydrochloride) 500 mg is an essential amino acid that must be obtained through diet or supplementation. Lysine is required for the production of all proteins in the body and is necessary for the maintenance of the structural proteins collagen and elastin, which form all connective tissue, such as skin, tendons and bones. Lysine is also a precursor to carnitine, which is needed for fat metabolism and energy production. In addition, lysine can help support a healthy immune system and proper cardiovascular function.