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Phenylalanine occurs in two chemical forms: L-phenylalanine, a natural amino acid found in proteins; and its mirror image, D-phenylalanine, a form synthesized in a laboratory. Some research has involved the L-form, others the D-form, and still others a combination of the two known as DL-phenylalanine.
In the body, phenylalanine is converted into another amino acid called tyrosine . Tyrosine in turn is converted into L-dopa, norepinephrine, and epinephrine, three key neurotransmitters (chemicals that transmit signals between nerve cells). Because some antidepressants work by raising levels of norepinephrine, various forms of phenylalanine have been tried as a possible treatment for depression.
D-phenylalanine (but not L-phenylalanine) has been proposed to treat...
Small double-blind, comparative studies suggest (but do not prove) that both the D- and DL- forms of phenylalanine might be helpful for depression . 1 Weak and contradictory evidence has been used to advocate the use of D-phenylalanine as a general analgesic (pain relieving treatment). 2 3 4 Preliminary uncontrolled and double-blind studies found that L-phenylalanine may enhance the effectiveness of ultraviolet for vitiligo . 5 6 Highly preliminary evidence suggests that D-phenylalanine may be helpful for multiple sclerosis when combined with transcutaneous electrical nerve stimulation (TENS). 7 D-phenylalanine has also been proposed as a treatment for Parkinson's disease (but see Safety Issues below). 8 Although D- and DL- phenylalanine are marketed as...
The long-term safety of phenylalanine in any of its forms is not known. Both L- and D-phenylalanine must be avoided by those with the rare metabolic disease phenylketonuria (PKU).
The maximum safe dosages of phenylalanine have not been established for young children, pregnant or nursing women, or those with severe liver or kidney disease.
There are some indications that the combined use of phenylalanine with antipsychotic drugs might increase the risk of developing the long-term side effect known as tardive dyskinesia, or worsen symptoms in those who already have it. 9 Like other amino acids, phenylalanine may interfere with the absorption or action of the drug levodopa, which is used for Parkinson's disease . 10
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