1. Clinical Overview
Granisetron is a potent and highly selective serotonin 5-HT3 receptor antagonist used primarily for the prevention and treatment of chemotherapy-induced nausea and vomiting (CINV) and postoperative nausea and vomiting (PONV). It is a first-line antiemetic agent in the Indian clinical setting, valued for its efficacy, safety profile, and convenient dosing schedule.
| Onset | Duration | Bioavailability |
|---|---|---|
| Intravenous: Within 1-3 minutes. Oral: Within 30-60 minutes. | Approximately 24 hours following a single dose, allowing for once-daily dosing in most regimens. | Approximately 60% following oral administration. |
2. Mechanism of Action
Granisetron selectively and competitively blocks serotonin (5-HT3) receptors located peripherally on vagal nerve terminals in the gastrointestinal tract and centrally in the chemoreceptor trigger zone (CTZ) of the area postrema in the medulla oblongata. Chemotherapy and radiotherapy cause the release of serotonin from enterochromaffin cells in the gut, which activates 5-HT3 receptors on vagal afferents, initiating the vomiting reflex. By blocking these receptors, granisetron interrupts the signal to the vomiting center.
3. Indications & Uses
- Prevention of nausea and vomiting associated with initial and repeat courses of emetogenic cancer chemotherapy (including high-dose cisplatin).
- Prevention and treatment of postoperative nausea and vomiting (PONV).
4. Dosage & Administration
Adult Dosage: Chemotherapy-induced: A single 2 mg oral dose taken up to 1 hour before chemotherapy, or 1 mg twice daily (first dose 1 hour pre-chemo, second dose 12 hours later). Radiotherapy-induced: 2 mg once daily within 1 hour of radiotherapy. Postoperative: 1-2 mg as a single dose before induction of anesthesia or postoperatively.
Administration: Tablet can be taken with or without food. Swallow whole with a glass of water. For PONV, follow the anesthesiologist's/surgeon's specific timing instructions.
5. Side Effects
Common side effects may include:
- Headache (14-21%)
- Constipation (3-9%)
- Asthenia/Weakness
- Diarrhea
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Apomorphine | Severe hypotension and loss of consciousness. Contraindicated combination. | Major |
| Other QT-prolonging drugs (e.g., Class Ia/III antiarrhythmics, fluoroquinolones, macrolides, antipsychotics) | Additive risk of QT prolongation and cardiac arrhythmias. Monitor ECG. | Moderate |
| Strong CYP3A4 inducers (e.g., Rifampicin, Phenytoin, Carbamazepine, St. John's Wort) | May decrease granisetron plasma concentrations, potentially reducing efficacy. | Moderate |
| Serotonergic drugs (e.g., SSRIs, SNRIs, tramadol, triptans) | Theoretical increased risk of serotonin syndrome. Monitor for symptoms. | Moderate |
7. Patient Counselling
- DO take the tablet exactly as prescribed by your doctor, usually 1 hour before your chemotherapy session.
- DO inform your doctor if you are pregnant, planning pregnancy, or breastfeeding.
- DO inform your doctor about all other medicines you are taking, including herbal supplements.
- DONT take a double dose if you miss one. Take it as soon as you remember, but if it's almost time for your next dose, skip the missed dose.
- DONT stop taking other prescribed antiemetics unless advised by your doctor.
8. Toxicology & Storage
Overdose: No specific syndrome reported. Expected symptoms would be an extension of side effects: severe headache, severe constipation, dizziness, and potentially QT prolongation.
Storage: Store below 30°C. Protect from light and moisture. Keep the tablet in the original blister pack or container until use. Keep out of reach of children.