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. L-lysine works in balance with the amino acid L-arginine, where high levels of L-lysine decrease L-arginine levels. High levels of L-arginine in the body encourages the replication of herpes viruses such as those seen with cold sores.