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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 and in the adrenal medulla of the adrenal glands to dopamine, norepinephrine (noradrenaline), and epinephrine 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 co-factors (vitamins B6 and C) are included.106-114

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

Cancer patients should avoid taking L-phenylalanine and L-tyrosine. Certain cancers, such as melanoma, depend on these amino acids to fuel their growth.

Supplemental use of L-phenylalanine and L-tyrosine may raise or normalize blood pressure. Insomnia may occur from over-stimulation if taken too close to bedtime. Individuals with the rare metabolic disorder phenylketonuria should avoid phenylalanine. Those suffering from migraine headaches should also avoid L-phenylalanine and L-tyrosine because they form tyramine, a substance that may trigger migraines.