Onsett 8 tablets

Indications

• Nausea and vomiting induced by chemotherapy
• Nausea and vomiting associated with radiotherapy
• Postoperative nausea and vomiting
• Nausea and vomiting in patients with gastroenteritis

Dosage

Children: Refer to the BNF for Children for appropriate paediatric dosing.

Adults: Refer to the BNF for specific dosing guidelines.

Mechanism of action

Ondansetron exerts its antiemetic effects by selectively antagonizing the 5-HT3 receptor subtype, which is found in the vagus nerve and the central nervous system. By inhibiting the binding of serotonin to these receptors, ondansetron prevents the activation of the vomiting center in the brain and reduces the perception of nausea.

Pharmacodynamics

Ondansetron shows a high affinity for 5-HT3 receptors, leading to effective inhibition of serotonin-induced nausea and vomiting. The drug can also exhibit some affinity for other serotonin receptors, but its primary action is at the 5-HT3 subtype. The onset of action is typically within 30 minutes when administered intravenously, and the duration of action can last for several hours, making it suitable for managing acute episodes of nausea and vomiting.

Pharmacokinetics

Ondansetron is rapidly absorbed after oral administration, with peak plasma concentrations occurring within 1 to 2 hours. It has a bioavailability of approximately 60-70% due to first-pass metabolism. The drug is primarily metabolized in the liver by cytochrome P450 enzymes, particularly CYP3A4, CYP2D6, and CYP1A2, leading to various metabolites, which are excreted via the urine. The elimination half-life of ondansetron is about 3-6 hours, and it is excreted mainly as metabolites, with less than 5% of the dose recovered as unchanged drug.

Contra-indications

• Hypersensitivity to ondansetron or any of its components
• Concomitant use with apomorphine due to the risk of severe hypotension

Adverse effects

• Headache
• Constipation
• Diarrhea
• Dizziness
• Fatigue
• Transient visual disturbances
• QT prolongation

Interactions

• Caution with drugs that prolong the QT interval
• Caution with serotonergic drugs due to risk of serotonin syndrome
• May affect the metabolism of drugs metabolized by CYP3A4

Precautions

• Use with caution in patients with electrolyte imbalances
• Monitor ECG in patients with a history of cardiac disease
• Caution in patients with hepatic impairment

Pregnancy

Ondansetron should only be used during pregnancy if the potential benefit justifies the potential risk to the fetus. Limited human data are available.

Breast-feeding

Ondansetron is excreted in human milk. Caution is advised when administering to breastfeeding women.

Storage

Store at room temperature, away from moisture and heat. Protect from light.

Formulations

• Tablets
• Oral solution
• Injection