3-(3,4-Dihydroxyphenyl)propanoic acid in which the hydrogens alpha- to the carboxyl group are substituted by hydrazinyl and methyl groups (S-configuration). Carbidopa is a dopa decarboxylase inhibitor, so prevents conversion of levodopa to dopamine. It has no antiparkinson activity by itself, but is used (commonly as its hydrate) in the management of Parkinson's disease to reduce peripheral adverse effects of levodopa.