Regarding the mechanism of action and clinical use of antiemetics, all of the following statements are correct EXCEPT:

Explanation

## Mechanism and Classification of Antiemetics ### Correct Statements **Key Point:** Metoclopramide is a dopamine antagonist that blocks D2 receptors in the chemoreceptor trigger zone and enhances gastric motility through acetylcholine effects on the stomach. **High-Yield:** 5-HT3 receptor antagonists (ondansetron, granisetron) block serotonin at both peripheral (gut vagal afferents) and central (chemoreceptor trigger zone) sites, making them highly effective for chemotherapy-induced nausea and vomiting (CINV). **Clinical Pearl:** Dexamethasone is a glucocorticoid that potentiates the antiemetic action of 5-HT3 antagonists and neurokinin-1 (NK1) antagonists in highly emetogenic chemotherapy regimens, often used as an adjunct in combination antiemetic protocols. ### The Incorrect Statement **Warning:** Prochlorperazine is NOT a 5-HT4 agonist. It is a **phenothiazine derivative** — a dopamine antagonist (D2 blocker) — similar to metoclopramide. Because of its dopamine antagonism, prochlorperazine carries a significant risk of **extrapyramidal side effects** (dystonia, akathisia, tardive dyskinesia), not a minimal risk. **Mnemonic:** **PHEN** = **Phenothiazine** = **Dopamine antagonist** = **Extrapyramidal risk** ### Comparison Table | Drug | Drug Class | Mechanism | Primary Use | Key Side Effect | | --- | --- | --- | --- | --- | | Metoclopramide | Benzamide | D2 antagonist + acetylcholine agonist | Gastric dysmotility, PONV | Extrapyramidal effects, tardive dyskinesia | | Prochlorperazine | Phenothiazine | D2 antagonist | PONV, migraine-associated nausea | **Extrapyramidal effects** (dystonia, akathisia) | | Ondansetron | 5-HT3 antagonist | Serotonin receptor blockade | CINV, PONV | Headache, constipation | | Dexamethasone | Corticosteroid | Anti-inflammatory, enhances other antiemetics | Adjunct in CINV | Hyperglycemia, immunosuppression |