What is the role of carbidopa in the carbidopa-levodopa combination?

• A. Carbidopa directly inhibits dopamine receptors.
• B. Levodopa increases serotonin in the brain.
• C. Levodopa is converted to dopamine in the brain, and carbidopa prevents peripheral breakdown of levodopa.
• D. Levodopa cures Parkinson disease.

Answer: (C)

Carbidopa’s job is to inhibit an enzyme outside the brain that would otherwise break down levodopa before it can reach the brain. By blocking peripheral decarboxylase, carbidopa prevents levodopa from being converted to dopamine in the rest of the body, which keeps more levodopa available to cross into the brain. Once inside the brain, levodopa is converted to dopamine, helping to relieve Parkinsonian symptoms. Carbidopa itself stays mostly outside the brain, so it doesn’t directly affect dopamine production in the brain, and it doesn’t cure the disease. The combination therefore improves symptom control while reducing peripheral side effects like nausea and cardiovascular effects that occur when too much dopamine is formed outside the brain.