Ondansetron

# Ondansetron ## Overview - **Classification**: 5-HT3 receptor antagonist, antiemetic. - **Mechanism**: Selectively blocks serotonin 5-HT3 receptors on vagal nerve terminals and in the chemoreceptor trigger zone. ## Primary Indications 1. **Chemotherapy-induced nausea and vomiting (CINV)** - Prevention and treatment. 2. **Postoperative nausea and vomiting (PONV)** - Prevention and treatment. 3. **Radiation-induced nausea and vomiting (RINV)** - Prevention. ## Adult Dosing ### Standard Dosing **Chemotherapy-Induced Nausea and Vomiting (CINV)** - **Highly Emetogenic CINV** - **Dose**: **0.15 mg/kg IV** (up to **16 mg** per dose) - **Frequency**: Administer 30 min before chemo, then 4 & 8 hours after first dose (total 3 doses). - **Route**: Intravenous (IV) - **Moderately Emetogenic CINV** - **Dose**: **8 mg IV** or **8 mg PO** - **Frequency**: Single dose before chemo, or **8 mg PO** q8h up to 2 days after chemo. - **Route**: IV or Oral (PO) **Postoperative Nausea and Vomiting (PONV)** - **Prevention** - **Dose**: **4 mg IV** or **16 mg PO** - **Frequency**: Single dose immediately before anesthesia induction (IV) or 1 hour pre-op (PO). - **Route**: IV or PO - **Treatment** - **Dose**: **4 mg IV** - **Frequency**: Single dose - **Route**: IV **Radiation-Induced Nausea and Vomiting (RINV)** - **Dose**: **8 mg PO** - **Frequency**: 1-2 hours before radiotherapy, then q8h for 1-2 days after. - **Route**: PO ### Dose Adjustments - **Renal Impairment**: No dose adjustment typically required. - **Hepatic Impairment**: **Severe hepatic impairment**: Max total daily dose **8 mg**. - **Elderly Patients**: No general adjustment, but consider increased risk of QT prolongation. ## Pediatric Dosing ### Neonates (0-28 days) - **Dose**: Not routinely recommended. - **Special Notes**: Limited data, potential for QT prolongation in a vulnerable population. Avoid if possible. ### Infants (1-12 months) - **PONV (≥1 month)**: - **Dose**: **0.1 mg/kg IV** - **Maximum**: **4 mg** per dose - **Frequency**: Single dose pre- or post-procedure. - **CINV (≥6 months)**: - **Dose**: **0.15 mg/kg IV** - **Maximum**: **8 mg** per dose - **Frequency**: Administer 30 min before chemo, then 4 & 8 hours after first dose (total 3 doses). ### Children (1-12 years) - **CINV (≥6 months)**: - **Dose**: **0.15 mg/kg IV** - **Maximum**: **16 mg** per dose - **Frequency**: Administer 30 min before chemo, then 4 & 8 hours after first dose (total 3 doses). - **Oral Option (for those >12 kg)**: **4 mg PO** 30 min before chemo, then 4 & 8 hours after. - **PONV (≥1 month)**: - **Dose (<40 kg)**: **0.1 mg/kg IV** - **Dose (>40 kg)**: **4 mg IV** - **Maximum**: **4 mg** per dose - **Frequency**: Single dose pre- or post-procedure. ### Adolescents (13-18 years) - **Dose**: Adult dosing applies. - **Maximum**: Refer to adult maximum doses. ## Safety Information ### Contraindications - **Absolute**: Concomitant use with **apomorphine** (severe hypotension). - **Absolute**: Known **hypersensitivity** to ondansetron. - **Absolute**: Patients with **congenital long QT syndrome**. ### Common Adverse Effects - **Very Common (>10%)**: Headache, constipation. - **Common (1-10%)**: Diarrhea, fatigue, dizziness, dry mouth. - **Serious but Rare**: QT prolongation, Torsades de Pointes, Serotonin Syndrome, hypersensitivity reactions, liver enzyme elevation. ### Key Drug Interactions - **Apomorphine**: Life-threatening **severe hypotension** and loss of consciousness. **Concomitant use contraindicated.** - **QT-prolonging drugs**: Increased risk of **QT prolongation** and Torsades de Pointes. Monitor ECG (e.g., amiodarone, quinidine, sotalol, certain antipsychotics). - **Serotonergic drugs (SSRIs, SNRIs, MAOIs, TCAs, triptans, fentanyl, tramadol)**: Increased risk of **Serotonin Syndrome**. Monitor for mental status changes, autonomic instability, neuromuscular abnormalities, GI symptoms. - **Tramadol**: May reduce tramadol's analgesic effect. ## Monitoring & Follow-up - **Before Treatment**: Assess baseline ECG (QTc) in patients with underlying cardiac conditions, electrolyte imbalances (K+, Mg2+), or on other QT-prolonging meds. - **During Treatment**: Monitor for signs of QT prolongation (e.g., palpitations, syncope). Monitor electrolytes in high-risk patients. Monitor for Serotonin Syndrome symptoms. - **Clinical Signs**: Watch for dizziness, blurred vision, constipation, or headache. ## Clinical Pearls - 💡 **Timing is Key**: Administer **before** the emetogenic event (chemo, surgery, radiation) for best efficacy. - 💡 **QT Risk**: Ondansetron can cause **dose-dependent QT prolongation**. Use caution in patients with electrolyte imbalances (hypokalemia, hypomagnesemia) or cardiac disease. - 💡 **Formulations**: Available as oral tablets, orally disintegrating tablets (ODT), oral solution, and IV injection. ODT can be useful for patients with dysphagia. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.