Generic Name: Ondansetron
Therapeutic Class: Anti-emetic (5-HT3 Receptor Antagonist)
Indications
Ondansetron is a potent anti-emetic used to manage and prevent nausea and vomiting across various clinical scenarios:
- Chemotherapy-Induced (CINV): Prevention of nausea and vomiting associated with highly or moderately emetogenic cancer chemotherapy.
- Radiotherapy-Induced (RINV): Prevention of nausea and vomiting in patients receiving total body irradiation or high-dose fractions to the abdomen.
- Post-operative (PONV): Prevention and treatment of post-operative nausea and vomiting.
- Gastroenteritis: Often used off-label in emergency settings to manage vomiting associated with acute viral gastroenteritis in children and adults.
- Pregnancy: Sometimes prescribed for severe cases of morning sickness or hyperemesis gravidarum when other treatments fail.
Dosage & Administration
Dosage varies significantly based on the indication and route of administration (Oral, IV, IM).
- Chemotherapy (Highly Emetogenic): A single 24 mg oral dose (three 8 mg tablets) administered 30 minutes before the start of single-day chemotherapy.
- Chemotherapy (Moderately Emetogenic): 8 mg orally twice daily. The first dose is given 30 minutes before chemotherapy, followed by a second dose 8 hours later. Continue 8 mg twice daily for 1–2 days after chemotherapy.
- Radiotherapy: 8 mg orally three times daily. The first dose should be taken 1–2 hours before radiotherapy.
- Post-operative: 16 mg orally 1 hour before induction of anesthesia, or 4 mg via slow IV/IM injection immediately before anesthesia.
- Pediatric (4–11 years): 4 mg orally three times daily.
- Hepatic Impairment: In patients with severe hepatic impairment (Child-Pugh score ≥ 10), the total daily dose should not exceed 8 mg.
Description & Pharmacokinetics
Ondansetron is a carbazole derivative and a highly selective antagonist of the 5-HT3 (serotonin) receptor.
Mechanism of Action: It works by blocking serotonin receptors both peripherally on vagal nerve terminals in the gastrointestinal tract and centrally in the **Chemoreceptor Trigger Zone (CTZ)** located in the area postrema of the brain. By inhibiting these receptors, it prevents the initiation of the vomiting reflex.
Pharmacokinetics:
Mechanism of Action: It works by blocking serotonin receptors both peripherally on vagal nerve terminals in the gastrointestinal tract and centrally in the **Chemoreceptor Trigger Zone (CTZ)** located in the area postrema of the brain. By inhibiting these receptors, it prevents the initiation of the vomiting reflex.
Pharmacokinetics:
- Absorption: Rapidly absorbed from the GI tract with a peak plasma concentration reached in 1.5 hours. Oral bioavailability is approximately 60%.
- Metabolism: Extensively metabolized in the liver via hydroxylation and subsequent glucuronide or sulfate conjugation (primarily involving CYP1A2, CYP2D6, and CYP3A4 enzymes).
- Excretion: Less than 5% of the absorbed dose is excreted unchanged in the urine. The elimination half-life is about 3 to 4 hours in healthy adults.
Side Effects
- Very Common: Headache (the most frequently reported side effect).
- Gastrointestinal: Constipation, diarrhea, and occasionally dry mouth.
- Neurological: Dizziness, fatigue, drowsiness, and rare reports of extrapyramidal reactions.
- Cardiovascular: Transient ECG changes, including **QT interval prolongation** and rare instances of Torsade de Pointes.
- General: Sensation of warmth or flushing, hiccups, and localized injection site reactions.
Extra Important Information: Usage Insights
- ODT Formulation: Orally Disintegrating Tablets (ODT) should be placed on the tongue to dissolve and swallowed with saliva. No water is needed.
- Serotonin Syndrome: Caution is advised when used with other serotonergic drugs (like SSRIs or SNRIs) due to the risk of Serotonin Syndrome.
- Bowel Activity: Ondansetron may mask a progressive ileus or gastric distension in patients following abdominal surgery.
Pregnancy & Lactation
- Pregnancy: Category B. While widely used, some studies suggest a slight increase in the risk of oral clefts if used in the first trimester. It should only be used if clearly needed.
- Lactation: It is unknown if ondansetron is excreted in human milk. Exercise caution when administering to nursing mothers.
Storage
Store tablets and oral solution at room temperature (15°C to 30°C). Protect from light and store in a dry place. Keep the ODT blisters in the original package until use.
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