Dopamine is synthesized by decarboxylation of the aminoacid L-dihydroxyphenylserine (L-DOPA) by the enzyme aromatic L-aminoacid decarboxylase, also known as DOPA decarboxylase. Patients with Parkinson's disease suffer nausea and vomiting when they receive treatment with L-DOPA. However, when L-DOPA is administered together with carbidopa, a reversible competitive inhibitor of DOPA decarboxylase that does not cross the blood brain barrier, nausea and vomiting are prevented. The investigators hypothesize that by blocking the conversion of DOPA to dopamine and thus preventing its increase in plasma, treatment with carbidopa will decrease nausea and vomiting in patients with FD.