1. Clinical Overview
Ondansetron is a potent, highly selective competitive antagonist of serotonin 5-HT3 receptors. It is a first-line antiemetic agent widely used in the Indian healthcare system for the prevention and treatment of chemotherapy-induced nausea and vomiting (CINV), radiotherapy-induced nausea and vomiting (RINV), and postoperative nausea and vomiting (PONV). Its efficacy, safety profile, and availability in multiple formulations (IV, oral tablets, oral dissolving films) make it a cornerstone of supportive care in oncology and anesthesia.
| Onset | Duration | Bioavailability |
|---|---|---|
| Oral: 30-60 minutes; Intravenous: Within 10-15 minutes. | Approximately 8-12 hours for a single dose. | Approximately 60% for oral tablets. |
2. Mechanism of Action
Ondansetron selectively blocks serotonin (5-HT3) receptors, both 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. This blockade inhibits the vomiting reflex initiated by serotonin release, which is a key mediator in chemotherapy, radiotherapy, and postoperative settings.
3. Indications & Uses
- Prevention of nausea and vomiting associated with highly and moderately emetogenic cancer chemotherapy.
- Prevention of postoperative nausea and vomiting (PONV).
- Prevention of nausea and vomiting associated with radiotherapy (total body irradiation or fractionated abdominal radiation).
4. Dosage & Administration
Adult Dosage: Chemotherapy-induced: 8 mg orally twice daily or 16-24 mg as a single dose prior to chemotherapy. Postoperative: 4-8 mg IV/IM/orally given prior to induction of anesthesia or postoperatively. Radiotherapy-induced: 8 mg orally three times daily.
Administration: Oral Tablet: Can be taken with or without food. Orally Disintegrating Tablet (ODT): Place on tongue, allow to dissolve, and swallow with saliva. Do not chew. IV Injection: Can be administered as a slow IV injection (over at least 30 seconds, preferably 2-5 minutes) or diluted in 50-100 mL of compatible IV fluid (NS, D5W, RL) and infused over 15 minutes.
5. Side Effects
Common side effects may include:
- Headache
- Constipation
- Diarrhea
- Dizziness
- Fatigue/Malaise
- Injection site reactions (warmth, redness)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Apomorphine | Profound hypotension and loss of consciousness. | Contraindicated |
| Drugs that prolong QT interval (e.g., Class Ia/III antiarrhythmics, macrolides, fluoroquinolones, tricyclic antidepressants) | Additive risk of QT prolongation and Torsades de Pointes. | Major |
| Serotonergic drugs (e.g., SSRIs, SNRIs, tramadol, fentanyl, lithium, MAOIs) | Increased risk of serotonin syndrome. | Major |
| Phenytoin, Carbamazepine, Rifampicin | Inducers of CYP3A4 may reduce ondansetron plasma concentrations, potentially decreasing efficacy. | Moderate |
| Cimetidine | May inhibit ondansetron metabolism, potentially increasing plasma levels. | Minor |
7. Patient Counselling
- DO take the medication as prescribed, usually 30 minutes before chemotherapy or surgery.
- DO inform your doctor if you are pregnant, planning pregnancy, or breastfeeding.
- DO inform your doctor about all other medications you are taking, including herbal supplements.
- DONT take a double dose if you miss one. Take the next dose at the scheduled time.
- DONT chew or crush the orally disintegrating tablet (ODT). Place it on your tongue and let it dissolve.
8. Toxicology & Storage
Overdose: Manifestations may include sudden transient visual disturbances (e.g., blindness), severe constipation, hypotension, dizziness, and syncope. At very high doses, ECG changes including QT prolongation may occur.
Storage: Store below 30°C. Protect from light and moisture. Keep orally disintegrating tablets in the blister pack until use. Keep all medicines out of reach of children.