L-phenylalanine is an essential amino acid that can be converted to L-tyrosine in the liver. L-tyrosine can be converted by neurons 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. 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 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 or L-tyrosine.

L-tyrosine is also a precursor of two thyroid hormones. D,L-phenylalanine is a 50/50 mixture of its two stereoisomers.

May cause anxiety, high blood pressure, and headache. Do not use if you have phenylketonuria. Do not use phenylalanine or tyrosine if you have cancer or muscular dystrophy. Because phenylalanine can elevate blood pressure in hypertensives, consult your healthcare practitioner if you have hypertension. Insomnia may occur from overstimulation if taken too close to bedtime.