A patient on long-term levodopa therapy for Parkinson disease develops nausea and vomiting. The physician prescribes an antiemetic that does not worsen parkinsonism. Which antiemetic is appropriate and why?

• A Domperidone because it is a peripheral D2 antagonist that does not cross the blood-brain barrier ✓
• B Metoclopramide because it acts only on 5-HT4 receptors in the gut
• C Ondansetron because it is a D2 antagonist acting on the chemoreceptor trigger zone
• D Prochlorperazine because it selectively blocks 5-HT3 receptors in the area postrema

Explanation

Domperidone is a peripheral dopamine D2 receptor antagonist that does not cross the blood-brain barrier, so it does not worsen parkinsonism. Metoclopramide and prochlorperazine cross the BBB and block central D2 receptors, exacerbating parkinsonism. Ondansetron is a 5-HT3 antagonist, not a D2 antagonist.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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