Phenylalanine is an essential amino acid because the body cannot produce it.  Dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk. Phenylalanine is used for depression, ADHD, and chronically painful ailments. It has also been known to help those with vitiligo.  Phenylalanine is an amino acid that comes in three forms: D-phenylalanine, L-phenylalanine, and the mix made in the laboratory called DL-phenylalanine.  Dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk. Phenylalanine is used for depression, ADHD, and chronically painful ailments. It has also been known to help those with vitiligo.