Ondansetron

Ondansetron is a medication used to prevent nausea and vomiting. It is a selective antagonist of the 5-HT3 receptor, acting both in the central nervous system at the chemoreceptor trigger zone and peripherally on gut vagal afferents to reduce the emetic response.

It is indicated for the prevention of nausea and vomiting associated with chemotherapy, radiation therapy, and after surgery, and is also used for postoperative nausea and vomiting. It is available in several forms, including oral tablets, orally disintegrating tablets, an oral solution, and an intravenous injection.

Ondansetron is well absorbed orally and has about 60% bioavailability. It is extensively metabolized in the liver, primarily by the cytochrome P450 enzymes CYP3A4, with contributions from CYP2D6 and CYP1A2. The elimination half-life in adults is typically 3 to 6 hours. Dosing depends on age, indication, and route; common regimens include a single 4 mg dose for postoperative prophylaxis and 4–8 mg for chemotherapy-induced nausea and vomiting, with adjustments for children and liver impairment.

Common adverse effects are headache, dizziness, and constipation. More serious but rare risks include QT interval prolongation and torsades de pointes, particularly with high intravenous doses or in patients with electrolyte abnormalities or other QT-prolonging factors. Caution is advised in patients with congenital long QT syndrome. The drug is contraindicated with apomorphine and should be used during pregnancy only when clearly needed; it can pass into breast milk. Potential drug interactions include other serotonergic agents and medicines that prolong the QT interval.