D-phenylalanine is a non-protein amino acid related to the essential amino acid L-phenylalanine. D- and L-phenylalanine are structural mirror images of each other. A small amount of D-phenylalanine may be converted to L-phenylalanine in the body. D-phenylalanine is reported to inhibit an enzyme that degrades enkephalins, which are naturally occurring substances in the body that belong to the class of compounds known as "endorphins". Aids in the relief of minor pain.