Indications
Ondansetron is indicated for:
Prevention of nausea and vomiting associated with highly emetogenic cancer chemotherapy
Prevention of nausea and vomiting associated with radiotherapy
Prevention of post operative nausea and vomiting
Therapeutic Class
Anti-emetic drugs
Pharmacology
Ondansetron is a selective 5-HT3 receptor antagonist. While its mechanism of action has not been fully characterized, ondansetron is not a dopamine-receptor antagonist. Serotonin receptors of the 5-HT3 type are present both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone of the area postrema. It is not certain whether ondansetron’s antiemetic action is mediated centrally, peripherally, or in both sites. However, cytotoxic chemotherapy appears to be associated with release of serotonin from the enterochromaffin cells of the small intestine.
Dosage & Administration
Prevention of chemotherapy induced nausea & vomiting (CINV):
Adult-
Tablet and oral solution: The recommended adult oral dosage of Ondansetron is 24 mg given as three 8 mg tablets in highly emetogenic chemotherapy. In case of moderately emetogenic chemotherapy the oral dose is one 8 mg Ondansetron tablet or 10 ml of Ondansetron oral solution given twice daily.
Injection: The recommended i.v. dose of Ondansetron is a single 32 mg dose or three 0.15 mg/kg doses. A single 32 mg dose is infused over 15 minutes beginning 30 minutes before the start of emetogenic chemotherapy. Subsequent doses (0.15 mg/kg) are administered 4 and 8 hours after the first dose of Ondansetron.
Suppository: The recommended adult dose is one 16 mg suppository 1-2 hours before treatment. Ondansetron should be continued for upto 5 days after a course of treatment.The recommended dose is one suppository daily.
Pediatric patients-
Tablet and oral solution: for pediatric patients 4 through 11 years of age the dosage is one 4 mg Ondansetron tablet or 5ml of Ondansetron solution should be administered 3 times a day for 1 to 2 days after completion of chemotherapy.
Injection: the dosage in pediatric patients 4 to 18 years of age should three 0.15-mg/kg doses.
Suppository:Not recommended.

Radiotherapy induced nausea and vomiting:
Adult-
Tablet and oral solution: the recommended oral dosage is one 8mg Ondansetron tablet or 10ml of Ondansetron oral solution given 3 times daily.

Post operative nausea & vomiting (PONV):

Adult-
Tablet and oral solution: The recommended dosage is 16 mg given as two 8 mg Ondansetron tablets or 20 ml of Ondansetron oral solution 1hour before induction of anesthesia.
Injection: The recommended I.V. dosage of Ondansetron for adults is 4 mg undiluted administered intravenously in not less than 30 seconds, preferably over 2 to 5 minutes, immediately before induction of anesthesia, or postoperatively if the patient experiences nausea and/or vomiting occurring shortly after surgery. Alternatively, 4 mg undiluted may be administered intramuscularly as a single injection for adults. In patients who do not achieve adequate control of postoperative nausea and vomiting following a single, prophylactic, preinduction, I.V. dose of Ondansetron 4 mg, administration of a second I.V. dose of 4 mg Ondansetron postoperatively does not provide additional control of nausea and vomiting.
Suppository: The recommended adult dose is one 16 mg suppository 1-2 hours before treatment. Ondansetron should be continued for upto 5 days after a course of treatment.The recommended dose is one suppository daily.
Pediatric patients-
Injection: The recommended I.V. dosage of Ondansetron for pediatric patients (2 to 12 years of age) is a single 0.1-mg/kg dose for pediatric patients weighing 40 kg or less, or a single 4 mg dose for pediatric patients weighing more than 40 kg. The rate of administration should not be less than 30 seconds, preferably over 2 to 5 minutes. Little information is available about dosage in pediatric patients younger than 2 years of age.
Suppository: Not recommended.
Interaction
In patients treated with potent inducers of CYP3A4 (i.e Phenytoin, Carbamazepine or Rifampicin), the oral clearance of Ondansetron was increased and Ondansetron blood concentrations were decreased. Data from small studies indicate that Ondansetron may reduce the analgesic effect of tramadol.
Contraindications
Ondansetron is contraindicated in patients with known hypersensitivity to the drug.
Side Effects
The most common adverse effects include headache, constipation, diarrhea. In chemotherapy induced nausea and vomiting rash has occurred in approximately 1% of patients receiving Ondansetron. Blurred vision, chest pain with or without ST segment depression, cardiac arrhythmias, hypotension and bradycardia have been rarely reported.