L-phenylalanine is an essential amino acid that can be converted to L-tyrosine in the liver. L-tyrosine can be converted in the brain and in the adrenal glands to dopamine, norepinephrine, and epinephrine (adrenaline) hormones that 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. The conversion step from L-tyrosine to norepinephrine may be enhanced if the co-factors (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 with L-phenylalanine.

D,L-phenylalanine is a 50/50 mixture of its two stereoisomers.