Pyridoxal phosphate ( vitamin B 6) is a required cofactor in this reaction, and may occasionally be administered along with l-DOPA, usually in the form of pyridoxine. Once l-DOPA has entered the central nervous system, it is converted into dopamine by the enzyme aromatic l-amino acid decarboxylase, also known as DOPA decarboxylase. Speech, swallowing disorders, postural instability and freezing gait are the least responsive symptoms. Bradykinesia and rigidity are the most responsive symptoms while tremors are less responsive to levodopa therapy. The therapeutic efficacy is different for different kinds of symptoms. Thus, l-DOPA is used to increase dopamine concentrations in the treatment of Parkinson's disease, Parkinsonism, dopamine-responsive dystonia and Parkinson-plus syndrome.

L-DOPA crosses the protective blood–brain barrier, whereas dopamine itself cannot. The enantiomeric purity of l-DOPA may be analyzed by determination of the optical rotation or by chiral thin-layer chromatography. As is true for many molecules, the human body produces only one of these isomers (the l-DOPA form). L-DOPA has a counterpart with opposite chirality, d-DOPA. As a drug, it is used in the clinical treatment of Parkinson's disease and dopamine-responsive dystonia. l-DOPA can be manufactured and in its pure form is sold as a psychoactive drug with the INN levodopa trade names include Sinemet, Pharmacopa, Atamet, and Stalevo. Furthermore, l-DOPA itself mediates neurotrophic factor release by the brain and CNS. l-DOPA is the precursor to the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline), which are collectively known as catecholamines. Humans, as well as a portion of the other animals that utilize l-DOPA, make it via biosynthesis from the amino acid l-tyrosine. L-DOPA, also known as levodopa and l-3,4-dihydroxyphenylalanine, is an amino acid that is made and used as part of the normal biology of some plants and animals, including humans.

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