Levodopa is a dopamine precursor which is administered to increase levels of dopamine in the Central Nervous System. However, most levodopa is decarboxylated to dopamine before it reaches the brain, and dopamine is unable to cross the blood-brain barrier. This means less dopamine is available to the CNS, and excess dopamine is circulated in extracerebral tissues, causing major adverse effects.
Benserazide inhibits this decarboxylation, allowing the levodopa to enter the brain instead. Because benserazide does not enter the CNS, DOPA decarboxylase is uninhibited there and metabolizes the levadopa into useful dopamine. Adverse effects caused by extracerebral dopamine, such as nausea and arrhythmia, are minimized. However, benserazide cannot reduce the CNS-mediated side effects of levodopa, particularly dyskinesia.
Benserazide has little therapeutic effect on its own, and is only administered in combination with levodopa.
Levodopa/benserazide/olanzapine: "Dropped head syndrome", parkinsonism and neuromuscular disorders in an elderly partient: case report
Nov 22, 2008; A 72-year-old woman developed "dropped head syndrome" and parkinsonism, while receiving olanzapine for suspected psychotic...