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Ondosteron

Ondansetron is a selective 5-HT3 (serotonin) receptor antagonist used primarily to prevent nausea and vomiting - especially chemotherapy-induced, radiation-induced, and post-operative nausea/vomiting (PONV). It works by blocking serotonin receptors in the gut and the chemoreceptor trigger zone (CTZ) in the brain.
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Here is a full drug profile for Ondansetron (brand name: Zofran), based on authoritative medical textbooks and clinical sources:

Ondansetron (Zofran)

Drug Class

Selective 5-HT3 (serotonin) receptor antagonist - antiemetic

Mechanism of Action

Ondansetron works by selectively blocking 5-HT3 receptors at two key sites:
  1. Periphery - on visceral vagal afferent fibers in the gut (which detect emetogenic stimuli and signal the brain)
  2. Central - in the chemoreceptor trigger zone (CTZ) of the brainstem
By blocking these serotonin receptors, it interrupts the vomiting reflex pathway. Notably, it is NOT a dopamine-receptor antagonist (unlike metoclopramide or promethazine).
  • Lippincott Illustrated Reviews: Pharmacology, p. 1433

Indications

IndicationNotes
Chemotherapy-induced nausea & vomiting (CINV)First-line for highly and moderately emetogenic regimens
Radiation-induced nausea & vomitingStandard use
Post-operative nausea & vomiting (PONV)IV 4 mg or 8 mg
Undifferentiated N&V in the EDFirst-line for all age groups
Acute gastroenteritis (children)Reduces failure of oral rehydration, IV hydration need, hospitalization
Hyperemesis gravidarumOff-label; 4 mg PO/IV q8h (use with caution - see pregnancy notes)

Dosing

  • CINV (IV): 0.15 mg/kg IV infusion 30 minutes before chemotherapy
  • PONV (IV): 4 mg IV
  • Oral: 4-8 mg tablets; daily maximum capped due to QT concerns
  • Hepatic insufficiency: Dosage adjustment required (unlike other 5-HT3 antagonists in this class, ondansetron is the only one that requires this)

Pharmacokinetics

  • Metabolism: Extensively hepatic, via CYP3A4, CYP2D6, CYP1A2
  • Excretion: Urine
  • Drug interactions: CYP3A4 inducers (e.g., St. John's wort, rifampin) or inhibitors (e.g., clarithromycin) can alter clearance. Absolute contraindication with apomorphine (profound hypotension).

Side Effects

  • Headache (most common)
  • Constipation
  • QT interval prolongation (dose-dependent) - risk of Torsade de Pointes at high doses
  • Fatigue, dizziness

Key Safety Warning: QT Prolongation

The FDA issued a safety advisory in 2011 regarding ondansetron and QT prolongation. Important points:
  • Risk is dose-dependent - the IV formulation maximum dose was limited as a result
  • Monitor ECG in patients with: congenital long QT syndrome, CHF, bradyarrhythmias, concomitant QT-prolonging drugs
  • Correct electrolyte abnormalities (hypokalemia, hypomagnesemia) before administration
  • Pediatric ventricular arrhythmia risk is approximately 3 in 100,000 patients, rising with underlying cardiac disease
  • Tintinalli's Emergency Medicine, p. 524

Contraindications / Precautions

  • Congenital long QT syndrome - avoid
  • Apomorphine co-administration - absolute contraindication (profound hypotension)
  • Pregnancy: Evidence is mixed - some studies suggest possible association with fetal anomalies; use cautiously and only when benefits outweigh risks
  • Children under 4 years - use with caution

Comparison Within the 5-HT3 Class

Other drugs in this class include dolasetron, granisetron, and palonosetron. Key distinctions:
  • Ondansetron + granisetron prevent emesis in ~50-60% of cisplatin-treated patients
  • Palonosetron has a much longer half-life (~42 hours) - single dosing usually sufficient
  • Only ondansetron requires dose adjustment in hepatic insufficiency
  • IV dolasetron was removed from the market due to QT concerns; IV ondansetron maximum dose was restricted
  • Lippincott Illustrated Reviews: Pharmacology, p. 1433

Clinical Pearls

  • No evidence of superiority over other antiemetics (metoclopramide, prochlorperazine) for undifferentiated ED nausea, but preferred due to better side-effect profile (no extrapyramidal effects, less sedation)
  • For CINV with highly emetogenic regimens, combine with a NK1 antagonist (aprepitant) + dexamethasone for optimal coverage, especially for delayed-phase CINV (>24 hours post-chemo)
  • In acute gastroenteritis in children, a 2016 meta-analysis showed ondansetron improved cessation of vomiting at 1 hour and reduced need for IV hydration vs. placebo
  • Does not induce or inhibit the hepatic CYP450 system itself
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