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