WHEN LEVODOPA IS ADMINISTERED ORALLY IT IS RAPIDLY DECARBOXYLATED TO DOPAMINE IN EXTRACEREBRAL TISSUES SO THAT ONLY A SMALL PORTION OF A GIVEN DOSE IS TRANSPORTED UNCHANGED TO THE CENTRAL NERVOUS SYSTEM. FOR THIS REASON, LARGE DOSES OF LEVODOPA ARE REQUIRED FOR ADEQUATE THERAPEUTIC EFFECT AND THESE MAY OFTEN BE ACCOMPANIED BY NAUSEA AND OTHER ADVERSE REACTIONS. CARBIDOPA IS A PERIPHERAL DECARBOXYLASE INHIBITOR. IT INHIBITS THE PERIPHERAL DECARBOXYLATION OF LEVODOPA TO DOPAMINE &, EFFECTIVE BRAIN CONCENTRATIONS OF DOPAMINE ARE PRODUCED WITH LOWER DOSES OF LEVODOPA.. UNLIKE LEVODOPA, IT DOES NOT CROSS THE BLOOD-BRAIN BARRIER. CARBIDOPA HAS NOT BEEN SHOWN TO ENHANCE THE INTRINSIC EFFICACY OF LEVODOPA IN PARKINSONIAN SYNDROMES.