An Essential Amino Acid For A Healthy Nervous System! L-Phenylalanine is an essential amino acid that is necessary for the synthesis of proteins. 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. Note: Manufacturers continually change product specifications. While we try our best to keep product descriptions up to date, they do not necessarily reflect the latest information available from the manufacturer. We are not responsible for incorrect or outdated product descriptions and/or images.