What is the purpose of adding carbidopa to levodopa in Parkinson disease therapy?

• A. To increase central metabolism
• B. To reduce peripheral metabolism
• C. To decrease central side effects
• D. To raise blood pressure

Answer: (B)

The main idea is that carbidopa blocks the peripheral conversion of levodopa to dopamine. By inhibiting the enzyme responsible for decarboxylating levodopa outside the brain, more unchanged levodopa reaches the brain where it can be converted to dopamine to improve motor symptoms. This peripheral block also reduces dopamine-related side effects like nausea and orthostatic hypotension that come from peripheral dopamine. Carbidopa itself barely enters the brain, so central conversion of levodopa remains available, increasing CNS dopamine without boosting peripheral effects. So the purpose is to reduce peripheral metabolism of levodopa, enhancing brain delivery and tolerability.