Carbidopa / levodopa use in the restless legs syndrome

Levodopa works by increasing dopaminergic activity in the brain similar to dopamine agonists.  However, unlike dopamine agonists Levodopa is in a class of medication called dopamine precursors. Levodopa is a chemical precursor to dopamine.  Because dopamine itself cannot cross the blood-brain barrier when taken orally, levodopa is able to enter the brain, where brain cells convert it into dopamine to help make up for a deficiency reducing the RLS leg discomfort and urge to move sensations.

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In medications like levodopa, carbidopa is added as a peripheral decarboxylase inhibitor.  It prevents levodopa from being broken down in the bloodstream before it reaches the brain, allowing a much smaller dose to be effective and reducing side effects like nausea.