Paper I
2023 July (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Levodopa and carbidopa in parkinsonism

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Answer

Levodopa is combined with carbidopa (a peripheral dopa-decarboxylase inhibitor that does not cross the blood-brain barrier) to prevent peripheral conversion of levodopa to dopamine before it reaches the CNS. Dopamine itself cannot cross the blood-brain barrier, so levodopa (its precursor, which can cross) must be given — but without carbidopa, most of an oral dose would be decarboxylated peripherally, producing dopamine-mediated peripheral adverse effects (nausea, vomiting, hypotension) while very little reached the brain. Carbidopa co-administration increases the fraction of levodopa reaching the CNS and substantially reduces the peripheral dose and adverse-effect burden needed for the same central effect.

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