From the Manufacturer:
L-phenylalanine is an essential amino acid which can be converted to L-tyrosine by a complex biochemical process which takes place in the liver. L-tyrosine can be converted by neurons in the brain to dopamine and norepinephrine (noradrenaline), hormones which are depleted by stress, overwork and certain drugs. By replenishing norepinephrine in the brain, mental energy levels are enhanced and a feeling of contentment often occurs. Because of the liver conversion necessary for L-phenylalanine to have these effects, L-tyrosine is often faster acting. In addition, the conversion step from L-tyrosine to norepinephrine may be enhanced if the cofactors (vitamins B6 and C) are included. Cells in the adrenal medulla synthesize and secrete norepinephrine and epinephrine. Since both norepinephrine and epinephrine can cause smooth muscle (arterial) contraction, care with blood pressure should be taken when supplementing L-phenylalanine or L-tyrosine. L-tyrosine is also a precursor of two thyroid hormones. D-phenylalanine is unique among amino acids because the D-form (normally not found in life) has biological activity that is different from its L-stereoisomer. D, L-phenylalanine is a 50/50 mixture of the two stereoisomers.