Nervous System Free-Form Non-GMO A Dietary Supplement Vegetarian - Vegan Amino Acids Family Owned Since 1968 GMP Quality Assured L-Phenylalanine is an essential amino acid that is necessary for the synthesis of protein. Because L-Phenylalanine can be readily converted into the amino acid tyrosine, which forms part of the structure of the neurotransmitters dopamine and norepinephrine, it is important for the maintenance of a healthy nervous system.