Quest L-Lysine is an essential amino acid that the body does not make. This means that adequate levels need to be obtained from food and supplementation. L-lysine is a building block of protein structures within the body, especially muscle and skin protein structures and bones. L-lysine works in balance with the amino acid L-arginine, and helps to prevent a high L-arginine environment and decreases the occurrence of herpes viruses such as those seen with cold sores.