Carbidopa dosage modifies L-dopa induced side effects and blood levels of L-dopa and other amino acids in advanced parkinsonism.

Thirty-four patients with advanced Parkinson's disease showing intolerance to therapeutic doses of L-DOPA were treated with L-DOPA plus carbidopa at two different proportions. Ten patients preferred medication containing 10% carbidopa while 24 preferred 25% carbidopa. The increase of carbidopa reduced gastrointestinal disturbances and psychiatric side effects related to L-DOPA, but improvement of disability when measured according to objective tests was modest. Higher doses of carbidopa reduced peripheral DOPA metabolism and increased blood levels of tryptophan, 3-OM-DOPA and DOPA in relation to the administered dose.