Ondansetron

1 medicine

Ondansetron is a serotonin 5-HT3 receptor blocker used to prevent nausea and vomiting from chemotherapy, radiotherapy and surgery. It can prolong the heart's QT interval, so it needs caution in people with heart rhythm problems or low potassium or magnesium.

Zofran

Ondansetron

4/8mg

Zofran is a digestive health medication containing Ondansetron, available as 4/8mg tablets.

from $1.17 / tablet View

Key facts

  • Ondansetron (found in Zofran and generic versions) is a 5-HT3 receptor antagonist that blocks the nerve signals in the gut and brain that trigger vomiting.
  • It is used short-term, often as a single dose or for a few days around chemotherapy, radiotherapy, surgery, or acute gastroenteritis.
  • Ondansetron can prolong the QT interval, an electrical timing measure in the heart, raising the risk of a dangerous irregular heartbeat, especially at high doses, by injection, or in people with existing heart rhythm problems or low potassium or magnesium.
  • Seek urgent care for fainting, chest pain, or a pounding or irregular heartbeat while taking it.

What ondansetron treats

Ondansetron treats and prevents nausea and vomiting caused by cancer chemotherapy and radiotherapy, and nausea and vomiting after surgery. It is also used for vomiting linked to acute gastroenteritis, particularly in children. It is less reliable for motion sickness and does not settle nausea from opioid painkillers as consistently as some alternatives.

How ondansetron works

Serotonin released in the gut and in a vomiting centre in the brainstem binds to 5-HT3 receptors and triggers the urge to vomit. Ondansetron blocks these receptors, interrupting the signal so nausea and the vomiting reflex are suppressed.

Before you take it

  • Do not take ondansetron if you have a rare inherited heart rhythm disorder called congenital long QT syndrome.
  • Tell your prescriber about any heart disease, low blood potassium or magnesium, liver disease, or other medicines that affect heart rhythm.
  • Combining ondansetron with other serotonin-raising drugs, such as SSRIs, SNRIs, tramadol or triptans, can trigger serotonin syndrome, a rare but serious reaction involving agitation, high fever and muscle rigidity.
  • Tell your prescriber if you are pregnant; ondansetron is generally avoided in early pregnancy unless the benefit clearly outweighs the risk.

Side effects

Common effects include headache, constipation, tiredness and mild dizziness.

Seek urgent medical care for:

  • A fast, slow, or irregular heartbeat, or fainting.
  • Chest pain.
  • Signs of serotonin syndrome: agitation, sweating, tremor, high fever, or muscle twitching.
  • Swelling of the face or throat, or difficulty breathing.

Safety essentials

  • Ondansetron's QT-prolongation risk means doses are capped and used cautiously in anyone with heart disease or an electrolyte imbalance; correct low potassium or magnesium before starting where possible.
  • Avoid combining it with other QT-prolonging medicines, such as certain antibiotics and antipsychotics, unless a doctor has reviewed the combination.
  • Do not exceed the prescribed dose or frequency, particularly in children, where dosing is based on body weight.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.