Levodopa is converted to dopamine via the action of a naturally occurring enzyme called DOPA decarboxylase. This occurs both in the peripheral circulation and in the central nervous system after levodopa has crossed the blood brain barrier. Activation of central dopamine receptors improves the symptoms of Parkinson's disease; however, activation of peripheral dopamine receptors causes nausea and vomiting. For this reason levodopa is usually administered in combination with a DOPA decarboxylase inhibitor (DDCI), in this case carbidopa, which is very polar (and charged at physiologic pH) and cannot cross the blood brain barrier, however prevents peripheral conversion of levodopa to dopamine and thereby reduces the unwanted peripheral side effects of levodopa. Use of carbidopa also increases the quantity of levodopa in the bloodstream that is available to enter the brain.