Ondem (ondansetron)

# Ondem (ondansetron) ## Overview Ondansetron is a selective 5-HT3 receptor antagonist that inhibits nausea and vomiting by blocking serotonin receptors both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone. ## Primary Indications * Prevention of chemotherapy-induced nausea and vomiting (CINV). * Prevention/treatment of postoperative nausea and vomiting (PONV). * Prevention of nausea and vomiting associated with radiotherapy. ## Adult Dosing * **CINV (Highly Emetogenic):** 24 mg orally 30 minutes before chemotherapy. * **CINV (Moderately Emetogenic):** 8 mg orally every 12 hours, starting 30 minutes before chemotherapy; continue for 1–2 days post-chemo. * **PONV:** 16 mg orally 1 hour before induction of anesthesia; alternatively, 4 mg IV/IM administered at induction or postoperatively. ## Pediatric Dosing * **CINV (Ages 4–11):** 4 mg orally 30 minutes before chemotherapy, then 4 mg 4 and 8 hours after the first dose, then 4 mg every 8 hours for 1–2 days. * **PONV (Ages 1 month–12 years):** Weight-based dosing: 0.1 mg/kg (up to 4 mg) IV administered preoperatively or postoperatively. * *Note: Dosing can vary by institutional protocol; always verify pediatric weight-based orders.* ## Dose Adjustments * **Hepatic Impairment:** Maximum dose of 8 mg/day in patients with severe hepatic impairment (Child-Pugh score ≥10). * **Renal Impairment:** No dosage adjustment required. ## Contraindications * Known hypersensitivity to ondansetron or other 5-HT3 receptor antagonists. * Concomitant use of apomorphine (risk of profound hypotension and loss of consciousness). * Congenital long QT syndrome. ## Adverse Effects * **Common:** Headache, constipation, malaise, fatigue, diarrhea. * **Serious:** QT interval prolongation (dose-dependent), Torsades de Pointes, serotonin syndrome (when used with other serotonergic agents), and anaphylaxis. ## Key Drug Interactions * **QT Prolonging Agents:** Increased risk of cardiac arrhythmias (e.g., amiodarone, citalopram, antipsychotics). * **Serotonergic Agents:** Increased risk of serotonin syndrome (e.g., SSRIs, SNRIs, MAOIs, tramadol). * **CYP2D6/3A4 Inducers:** May decrease ondansetron plasma concentrations (e.g., rifampin, phenytoin). ## Monitoring * **ECG:** Consider monitoring in patients with electrolyte abnormalities (hypokalemia/hypomagnesemia), congestive heart failure, or those taking other QT-prolonging medications. * **Clinical:** Monitor for signs of serotonin syndrome (agitation, tachycardia, hyperreflexia). * **Hydration:** Monitor fluid/electrolyte status if vomiting persists. ## Clinical Pearls * **QT Risk:** Single IV doses should not exceed 16 mg. * **Administration:** Orally disintegrating tablets (ODT) should be placed on the tongue and allowed to dissolve; they are not intended for administration via enteral tubes. * **Efficacy:** Ondansetron is highly effective for chemotherapy and postoperative nausea but is less effective for nausea caused by motion sickness or delayed-phase CINV compared to other agents (e.g., NK1 antagonists). *** *Disclaimer: This information is for educational purposes only. Always verify current prescribing information, institutional protocols, and patient-specific factors via reliable resources such as Lexicomp or the official FDA package insert before prescribing or administering medication.*