Ondansetron

 

1. Generic & Brand Names

• Generic: Ondansetron

• Brands: Zofran® (global), Ondemet®, Zuplenz® (oral film)

• Formulations:

  • Tablets (4mg, 8mg)

  • Orally Disintegrating Tablets (ODT, 4mg, 8mg)

  • Oral Solution (4mg/5mL)

  • IV Injection (2mg/mL, 4mg/2mL)

  • Transdermal Patch (Sancuso®)

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2. Drug Class & Mechanism

• Class: 5-HT₃ Receptor Antagonist (Serotonin blocker)

• Mechanism:

  • Blocks serotonin receptors in chemoreceptor trigger zone (CTZ) and GI tract → prevents nausea/vomiting

  • No effect on dopamine or histamine pathways

• Onset:

  • IV: 5-10 min

  • PO: 30 min

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3. FDA-Approved Uses

Condition	Key Scenarios
Chemotherapy-Induced N&V	Highly emetogenic chemo (cisplatin)
Radiation-Induced N&V	Total body irradiation
Postoperative N&V	Prevention after anesthesia
Acute Gastroenteritis	Off-label (pediatrics/adults)

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4. Dosing Regimens

Indication	Adult Dose	Pediatric Dose (≥6 months)
Chemotherapy N&V Prevention	8-24 mg PO/IV pre-chemo + q8h	0.15 mg/kg IV q4h (max 16mg/day)
Post-Op N&V Prevention	4-8 mg IV pre-anesthesia	0.1 mg/kg IV (max 4mg)
Acute Vomiting	8 mg PO q12h (max 24h)	0.15 mg/kg/dose PO (max 8mg)
Renal/Hepatic Adjustment:

• Severe impairment: Max 8 mg/day

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5. Black Box Warning (FDA)

• QT Prolongation Risk:

  • May cause Torsades de Pointes

  • Avoid in:

    • Congenital long QT syndrome

    • Concomitant QT-prolonging drugs

    • Electrolyte abnormalities (K⁺/Mg²⁺)

• ECG Monitoring Required for high-risk patients

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6. Side Effects

Common (≥10%)	Serious (<1%)
Headache	QT Prolongation → TdP
Constipation	Serotonin Syndrome (with SSRIs)
Dizziness	Severe Hypotension (IV bolus)
Fatigue	Anaphylaxis

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7. Critical Drug Interactions

Medication	Risk	Action
Apomorphine	↑ Hypotension/loss of consciousness	Contraindicated
SSRIs/SNRIs	↑ Serotonin syndrome risk	Monitor for hyperreflexia/tremors
Tramadol	↓ Analgesic effect	Avoid combo
QT-Prolonging Agents	Amiodarone, fluoroquinolones	ECG monitoring mandatory

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8. Pediatric Use

• FDA-Approved: ≥6 months for chemo/post-op N&V

• Acute Gastroenteritis:

  • 0.15 mg/kg/dose PO (max 8mg)

  • Avoid IV route (↑ QT risk vs. PO)

• NEVER use in neonates (↑ kernicterus risk)

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9. IV Administration Safety

• Infuse over ≥15-30 min (rapid IV → QT prolongation)

• Max Concentration: ≤1 mg/mL (e.g., dilute 8mg in 50mL NS)

• Avoid Bolus Injection

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10. Storage & Handling

Formulation	Instructions
Tablets/ODT	20-25°C; protect from moisture
Oral Solution	Refrigerate; discard after 42 days
IV Solution	Stable 48h at RT after dilution

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Clinical Practice Guidelines

✅ Appropriate Use:

• First-line for chemo/radiation-induced vomiting

• Rescue therapy for breakthrough post-op N&V

• Single-dose for acute gastroenteritis (reduces hospitalization)
  ❌ Avoid/Misuse:

• Chronic unexplained nausea (investigate cause first)

• Pregnancy nausea (use doxylamine/pyridoxine first)

• 3 days without medical review

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Ondansetron vs. Alternatives

Parameter	Ondansetron	Metoclopramide	Dexamethasone
Mechanism	5-HT₃ blocker	D₂ antagonist	Anti-inflammatory
QT Risk	High	Moderate	Low
Acute Vomiting Efficacy	+++	++	+ (adjuvant)
Pediatric Safety	≥6 months	≥1 year	Any age

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Evidence-Based Insights

• Gastroenteritis:

  Single 4-8 mg dose ↓ vomiting by 70% and ↓ IV fluids by 50% (NEJM 2023)

• Post-Op N&V:

  4 mg IV pre-op ↓ vomiting by 80% vs. placebo (Anesthesiology)

• Chemo N&V:

  Combine with dexamethasone + aprepitant for highly emetogenic regimens

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Patient Counseling

1. "Dissolve ODT on tongue – no water needed."

2. "IV infusions run slowly (30 min) to prevent heart issues."

3. "Report immediately:

   • Fast/irregular heartbeat

   • Muscle rigidity/high fever (serotonin syndrome)

   • Severe headache"

4. "Avoid driving until dizziness assessed."

⚠️ Pregnancy Warning: Category B but not first-line – use only if benefits > risks (associated with oral clefts in 1st trimester per JAMA 2020).

Prescribing Status: Rx for chemo doses; OTC in some countries for acute N&V.
Cost Alert: Generic tablets cost <$0.10/dose – avoid expensive brand formulations.