Ondansetron

Is ondansetron safe?

Yes, for most people it is one of the gentlest anti-sickness drugs available. Ondansetron, first sold as Zofran, blocks serotonin signals that trigger vomiting, and it changed chemotherapy when it arrived in 1991. It is now used after surgery, in emergency departments and off-label for severe pregnancy sickness, with about 13.8 million US prescriptions in 2024 (ClinCalc). It does not sedate, is not addictive and is hard to overdose on.

We score it 82, level with azithromycin (82), which shares the same main caution: a small effect on heart rhythm. The other two things to know are serotonin syndromeSerotonin syndromeA reaction caused by too much serotonin in the body, usually from combining medicines that raise it. Symptoms range from shivering to high fever.More in the glossary, mostly when it is combined with antidepressants, and a small rise in cleft lip and palate when used in the first 12 weeks of pregnancy.

Does ondansetron affect the heart rhythm?

It can, depending on the dose. Ondansetron lengthens the QT intervalQT prolongationA delay in the heart's electrical recovery between beats, seen on an ECG. Some medicines cause it, and in rare cases it leads to a serious heart rhythm problem.More in the glossary, the time the heart takes to recharge between beats, and a long QT can rarely tip into torsade de pointes, a dangerous rhythm. In a study in healthy adults reviewed by the MHRA, 32 mg into a vein raised QTc by about 20 ms, while 8 mg raised it by about 6 ms, which is not usually a concern. In 2012 the MHRA limited single IV doses for chemotherapy to 16 mg.

The tablets you take at home sit at the low end of that range. The US label says to avoid ondansetron in people born with long QT syndrome and to check ECGs in those with low potassium or magnesium, heart failure, a slow heart rate, or other QT-prolonging drugs such as some antibiotics, antipsychotics and antidepressants like escitalopram.

What is the serotonin syndrome risk?

Serotonin syndromeSerotonin syndromeA reaction caused by too much serotonin in the body, usually from combining medicines that raise it. Symptoms range from shivering to high fever.More in the glossary means too much serotonin activity: agitation, sweating, fever, a fast heart rate, shaking, twitching and stiff muscles. The US label says cases have been reported with ondansetron alone but mostly with other serotonin drugs, including SSRIs such as sertraline, SNRIs, MAOIs, mirtazapine, fentanyl, lithium, tramadol and IV methylene blue. Some cases were fatal. Most happened in recovery rooms or infusion centres, where several such drugs are given together.

This does not mean people on antidepressants cannot take it; many do. It means your prescriber should know, and you should know the warning signs. One combination is banned outright: apomorphine, a Parkinson's drug, because together they can cause a severe fall in blood pressure and loss of consciousness.

Is ondansetron safe in pregnancy?

This is the question most people ask. Ondansetron is not licensed for pregnancy sickness, but it is widely used for hyperemesis gravidarum, severe vomiting that can cause dehydration and weight loss. The large studies agree on most points:

The MHRA judged the cleft finding robust after an EU review and advised in January 2020 that use in the first 12 weeks should be a joint decision after counselling. The US label calls the evidence inconsistent. Either way the absolute extra risk is about 3 in 10,000, and that has to be weighed against the harm of uncontrolled vomiting. A doctor or midwife can talk through first-line options before ondansetron.

Who should be careful

• People with long QT syndrome, heart failure, a slow heart rate, or low potassium or magnesium.
• Anyone on apomorphine (do not combine) or on several serotonin drugs.
• People with severe liver disease, who should take no more than 8 mg a day.
• After abdominal surgery: ondansetron can hide a developing bowel blockage, so report bloating, pain or no bowel movements.
• Households with small children: an overdose of more than 5 mg per kg has caused serotonin syndromeSerotonin syndromeA reaction caused by too much serotonin in the body, usually from combining medicines that raise it. Symptoms range from shivering to high fever.More in the glossary in toddlers.

If you need help with an overdose, call poison control (US 1-800-222-1222) or 111 in the UK.

How we scored it

Dependence (97), track record (93) and overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary (85) are strong, and serious side effects (82) are rare at tablet doses. Interactions (65) are the weak point because of the serotonin and QT combinations and the apomorphine ban. Regulatory warnings (68) reflect the 2012 IV dose limit and the 2020 pregnancy warning, with no boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary.