L-Phenylalanine, an essential amino acid, is a precursor of the amino acid tyrosine and a whole series of neurotransmitters and hormones. Phenylalanine is first converted to tyrosine, which becomes L-dopa, which converts to norepinephrine, a key neurotransmitter and an adrenal hormone. L-Phenylalanine is also a direct precursor of PEA (phenylethylamine), another important neurotransmitter.

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."

DL-Phenylalanine or DLPA, is a 50/50 mixture of the D- and L- forms of the amino acid phenylalanine (phenylalanine is one of the few aminos that can be utilized in its D- form). Researchers believe that D-phenylalanine (DPA) inhibits the action of an enzyme that breaks down endorphins and enkephalins. These are proteins which bind to opiate receptor sites in the brain, thereby influencing the perception of minor discomfort associated with normal physiological processes. By obstructing the action of this enzyme, DPA may extend the life of endorphins and enkephalins and their positive influence on comfort levels. L-Phenylalanine is the precursor to two neurotransmitters that help support an alert state.