## Mechanism of Metoclopramide **Key Point:** Metoclopramide is a benzamide derivative that acts primarily as a dopamine D2 receptor antagonist at the chemoreceptor trigger zone (CTZ), which lies outside the blood-brain barrier. ### Site of Action Metoclopramide blocks dopamine receptors in the CTZ, preventing activation of the vomiting centre. This mechanism makes it effective for chemotherapy-induced nausea and vomiting (CINV) and postoperative nausea and vomiting (PONV). ### Additional Properties - **Peripheral action:** Also enhances gastric motility via acetylcholine potentiation (anticholinesterase effect) - **Dual benefit:** Antiemetic + prokinetic agent - **Tardive dyskinesia risk:** Long-term use (>3 months) carries risk of irreversible movement disorder **High-Yield:** Metoclopramide is unique among antiemetics for combining dopamine antagonism with prokinetic activity. ### Comparison with Other Antiemetics | Drug Class | Mechanism | Primary Use | |---|---|---| | Dopamine antagonists (Metoclopramide) | D2 blockade at CTZ | CINV, PONV, gastroesophageal reflux | | 5-HT3 antagonists (Ondansetron) | 5-HT3 receptor blockade | CINV, PONV (highly selective) | | Anticholinergics (Scopolamine) | Muscarinic blockade | Motion sickness, postoperative nausea | | Corticosteroids (Dexamethasone) | Unknown mechanism | Adjunctive in CINV | [cite:KD Tripathi 8e Ch 47]
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