Adults

Chemotherapy-induced nausea and vomiting: 8 mg administered 30 minutes before chemotherapy, followed by 8 mg 8 hours after the first dose, and then 8 mg every 12 hours for 1 to 2 days after chemotherapy. Post-operative nausea and vomiting: 4 mg orally before anesthesia or as soon as nausea/vomiting begins.

Elderly

No dosage adjustment is generally required in elderly patients, but monitoring for QT prolongation is advisable, especially in patients with cardiac risk factors.

Renal_impairment

No dosage adjustment or special precautions are required for patients with renal impairment.

Onset

Approximately 30 minutes after oral administration.

Excretion

Approximately 5% of the dose is excreted unchanged in the urine. The major portion is excreted as metabolites in the urine and, to a lesser extent, in the feces.

Half life

The terminal elimination half-life is approximately 3-6 hours in adults. It may be prolonged in patients with severe hepatic impairment.

Absorption

Rapidly and completely absorbed from the gastrointestinal tract. Peak plasma concentrations are achieved approximately 1.5 hours after oral administration. Bioavailability is approximately 60% due to first-pass metabolism.

Metabolism

Extensively metabolized in the liver by cytochrome P450 enzymes (primarily CYP1A2, CYP2D6, CYP3A4) via hydroxylation, followed by glucuronide or sulfate conjugation. Less than 5% is excreted unchanged.

Tramadol

Reduced analgesic effect of tramadol has been reported with concomitant use. Ondansetron may inhibit CYP2D6, affecting tramadol metabolism.

Apomorphine

Concomitant use is contraindicated due to reports of profound hypotension and loss of consciousness.

QT prolonging drugs

Use with caution with other drugs known to prolong the QT interval (e.g., antiarrhythmics, antipsychotics) as it may increase the risk of arrhythmias.

Phenytoin, Carbamazepine, Rifampicin

These potent CYP3A4 inducers can increase the clearance of ondansetron, leading to reduced plasma concentrations.