How does the drug carbidopa affect levodopa?

Blocks breakdown: Carbidopa acts to block the enzyme aromatic-l-amino-acid decarboxylase which converts l-dopa to dopamine. Since it cannot get into the brain it works in the rest of the body (periphery). When given in combination with l-dopa in parkinson's disease, it inhibits the conversion of l-dopa to Dopamine in the body so that more is available to be converted in the brain where it is needed.