L-phenylalanine is an essential amino acid that plays a role in the biosynthesis of proteins. It is also involved in the production of certain neurotransmitters, including dopamine and norepinephrine. L-phenylalanine can be obtained from dietary sources or synthesized from other amino acids. It is used therapeutically to treat phenylketonuria, a genetic disorder that results in the accumulation of phenylalanine in the blood.