1. Clinical Overview
A fixed-dose combination (FDC) of a selective 5-HT3 receptor antagonist (Ondansetron) and a histamine H2-receptor antagonist (Ranitidine). It provides synergistic antiemetic and gastroprotective action, primarily used for the prevention and treatment of nausea and vomiting associated with chemotherapy, radiotherapy, and post-operative recovery, while also reducing gastric acid secretion to prevent associated mucosal irritation. This combination is particularly relevant in the Indian context for managing chemotherapy-induced nausea and vomiting (CINV) and post-operative nausea and vomiting (PONV) in patients with concomitant risk of acid-related disorders.
| Onset | Duration | Bioavailability |
|---|---|---|
| Ondansetron: Oral: 30-60 minutes; IV: 1-5 minutes. Ranitidine: Oral: 1-3 hours for peak acid suppression. | Ondansetron: 8-12 hours. Ranitidine: Oral: Up to 12 hours. | Ondansetron: Oral: Approximately 60% (subject to first-pass metabolism). Ranitidine: Oral: 50-60%. |
2. Mechanism of Action
The combination works via two distinct pathways. Ondansetron selectively blocks serotonin (5-hydroxytryptamine or 5-HT3) receptors peripherally on vagal nerve terminals in the gastrointestinal tract and centrally in the chemoreceptor trigger zone (CTZ) of the area postrema. This blockade inhibits the vomiting reflex. Ranitidine competitively inhibits histamine at H2 receptors of gastric parietal cells, leading to a marked reduction in basal and stimulated gastric acid secretion, volume, and hydrogen ion concentration.
3. Indications & Uses
- Prevention & treatment of nausea and vomiting induced by cytotoxic chemotherapy and radiotherapy.
- Prevention & treatment of post-operative nausea and vomiting (PONV).
4. Dosage & Administration
Adult Dosage: One tablet (Ondansetron 2mg + Ranitidine 150mg) twice daily, or as directed by the physician. For chemotherapy: Often started before chemotherapy. Max: Ondansetron 24mg/day; Ranitidine 300mg/day.
Administration: Oral administration. Can be taken with or without food. Tablet should be swallowed whole with a glass of water. For PONV prophylaxis, administer 1 hour before anesthesia. Antacids containing magnesium/aluminum hydroxide may reduce Ranitidine absorption; administer at least 2 hours apart.
5. Side Effects
Common side effects may include:
- Headache
- Constipation
- Diarrhea
- Dizziness
- Drowsiness
- Local injection site reactions (if IV form of individual drugs)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Apomorphine | Profound hypotension and loss of consciousness. | Contraindicated |
| Drugs prolonging QT interval (e.g., Amiodarone, Sotalol, Moxifloxacin, Antipsychotics) | Additive risk of QT prolongation and cardiac arrhythmias. | Major |
| Phenytoin, Carbamazepine, Rifampicin | Induce CYP450 enzymes, reducing Ondansetron plasma levels. | Moderate |
| CYP3A4 inhibitors (e.g., Ketoconazole, Erythromycin, Clarithromycin) | May increase Ondansetron plasma levels. | Moderate |
| Antacids, Sucralfate | Reduce absorption of Ranitidine. Administer 2 hours apart. | Moderate |
| Warfarin | Ranitidine may alter warfarin clearance; monitor INR. | Moderate |
| Midazolam, Triazolam | Ranitidine may increase bioavailability of these benzodiazepines. | Moderate |
| Procainamide | Ranitidine may reduce renal clearance of procainamide. | Moderate |
7. Patient Counselling
- Do take the tablet as prescribed, usually 1 hour before chemotherapy or surgery.
- Do inform your doctor about all other medicines, including OTC antacids and supplements.
- Do report any severe headache, constipation, or dizziness.
- Don't crush or chew the tablet.
- Don't take antacids within 2 hours of this medicine.
- Don't drive or operate heavy machinery if you feel dizzy.
8. Toxicology & Storage
Overdose: Ondansetron: Sudden transient blindness (IV), severe constipation, hypotension, dizziness. Ranitidine: Bradycardia, hypotension, vomiting, diarrhea, muscle cramps, confusion, delirium. Combined overdose may exaggerate CNS and CV effects.
Storage: Store below 30°C. Protect from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.