Twenty Parkinson disease patients with long-standing nausea and vomiting and with major and stable improvement in parkinsonian symptoms participated in a double-blind trial of carbidopa, a peripherally acting dopa decarboxylase inhibitor. Initially, half the patients, selected randomly, received carbidopa plus 20% of their previous levodopa dosage in fixed combination tablets containing levodopa:carbidopa in a 10:1 ratio. The other half received levodopa alone. Later, all patients received the combination tablets.
Throughout the study the only meaningful side effect was choreoathetosis. This occurred in four patients who had had this previously while taking levodopa alone.
All patients improved remarkably in nausea and vomiting, which persisted for the duration of the study. Most also showed mild to moderate improvement in the parkinsonian signs, symptoms, and functional states, as measured on three separate grading systems.
Markham CH, Diamond SG, Treciokas LJ. Carbidopa in Parkinson Disease and in Nausea and Vomiting of Levodopa. Arch Neurol. 1974;31(2):128–133. doi:10.1001/archneur.1974.00490380076010