Ondansetron (Perioperative)

How it works

Ondansetron selectively blocks 5-HT3 (serotonin) receptors centrally at the chemoreceptor trigger zone and peripherally on vagal afferents, interrupting the emetic reflex.

Prescribing in practice

• Ondansetron prolongs the QT interval — avoid or use with caution alongside other QT-prolonging drugs (including some anaesthetic agents), in congenital long QT syndrome, and correct hypokalaemia and hypomagnesaemia.
• It is most effective as part of a multimodal antiemetic strategy combining agents from different classes for high-risk patients rather than as a sole agent.
• Common effects include headache and constipation, and rare serotonin syndrome can occur with other serotonergic drugs.

Monitoring

Consider ECG and electrolyte checks in patients with cardiac risk factors or on other QT-prolonging therapy, and monitor for ongoing nausea requiring an additional class of antiemetic.

Counselling the patient

• This medicine helps stop sickness after your operation.
• Tell staff if you still feel sick, as a different anti-sickness drug can be added.
• Headache is a common, harmless side effect.

Evidence & guidelines

5-HT3 antagonists are guideline-recommended first-line agents for prevention of post-operative nausea and vomiting.