Ondansetron (Hyperemesis Gravidarum)

How it works

It blocks serotonin 5-HT3 receptors centrally and on vagal afferents in the gastrointestinal tract, interrupting the emetic reflex.

Prescribing in practice

• Use in the first trimester only after first-line antiemetics have failed, after a discussion of a small possible increase in the risk of orofacial clefts noted in some studies, in line with MHRA advice.
• It prolongs the QT interval, so caution is needed with electrolyte disturbance (common in hyperemesis) and other QT-prolonging drugs.
• Constipation is a common adverse effect.

Monitoring

Correct dehydration and electrolyte disturbances and consider ECG monitoring of the QT interval where risk factors are present.

Counselling the patient

• This is used when other anti-sickness medicines have not worked, after weighing the benefits and risks in early pregnancy.
• Constipation is common; maintain fluids and fibre as able.
• Report palpitations or fainting.

Evidence & guidelines

MHRA advises caution with ondansetron in the first trimester owing to a possible small increased risk of orofacial clefts; RCOG guidance positions it as a later-line antiemetic in pregnancy.