Levodopa

Levodopa, the immediate metabolic precursor of dopamine, does penetrate the blood brain barrier, where it is decarboxylated to dopamine. Levodopa is rapidly absorbed from the small intestine. Food will delay the appearance of levodopa in the plasma. It is extensively metabolized by peripheral dopa decarboxylase, hence given in combination with carbidopa, a peripheral dopa decarboxylase inhibitor.

When levodopa is given without carbidopa it causes vomiting (which is due to stimulation of emetic center to dopamine) and CVS disorder (tachycardia, ventricular extrasystoles, atrial fibrillation and due to increased catecholamine formation peripherally).