A hydrochloride that is the monohydrochloride salt of benserazide. An aromatic-L-amino-acid decarboxylase inhibitor (DOPA decarboxylase inhibitor) that does not enter the central nervous system, it is used as an adjunct to levodopa in the treatment of parkinsonism. By preventing the conversion of levodopa to dopamine in the periphery, it causes an increase in the amount of levodopa reaching the central nervous system and so reduces the required dose. Benserazide hydrochloride has no antiparkinson actions when given alone.