Per WebMD: Phenylalanine is an amino acid, a “building block” of protein. There are three forms of phenylalanine: D-phenylalanine, L-phenylalanine, and the mix made in the laboratory called DL-phenylalanine. D-phenylalanine is not an essential amino acid, and its role in people is not currently understood. L-phenylalanine is an essential amino acid and is the only form of phenylalanine found in proteins. Major dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk. Phenylalanine is used for depression, attention deficit-hyperactivity disorder (ADHD),Parkinson's disease, chronic pain, osteoarthritis, rheumatoid arthritis, alcohol withdrawal symptoms, and a skin disease called vitiligo. Some people apply it directly to the skin for vitiligo. How does it work? The body uses phenylalanine to make chemical messengers, but it is not clear how phenylalanine might work.