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Ondansetron: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Nov 6, 2019.

1. How it works

  • Ondansetron treats and prevents nausea and vomiting by an unknown mechanism, possibly by a direct effect on the CTZ (the area of the brain associated with vomiting), the vagus nerve or both. The neurotransmitter, serotonin, appears to play a role in ondansetron's effect.
  • Ondansetron belongs to the class of medicines known as 5-HT3 receptor antagonists.

2. Upsides

  • Effective for the prevention of nausea and vomiting associated with cancer chemotherapy, including highly emetogenic (a high probability of causing vomiting) chemotherapy.
  • May also be used to prevent nausea and vomiting associated with radiotherapy - this includes total body irradiation or single or multiple high-dose or daily fractions to the abdomen.
  • Effective at preventing postoperative nausea and vomiting.
  • Generic ondansetron is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Tiredness, a headache, drowsiness, dizziness, constipation, flushing, itching, slowed heart rate, raised body temperature, and diarrhea.
  • Rarely may cause hiccups, blurred vision or vision loss, a rash or liver disturbances.
  • Interaction or overdosage may cause serotonin syndrome (symptoms include agitation, hallucinations, fast heart rate, dizziness, muscle tremor, nausea, vomiting, diarrhea).
  • May interact with some medications including apomorphine (may cause a profound drop in blood pressure and loss of consciousness), and drugs metabolized by CYP3A4 (although few clinically significant interactions have been reported).
  • May not be suitable for some people including those with liver disease or a personal or family history of long QT syndrome.
  • Orally disintegrating ondansetron tablets may contain phenylalanine. Tell your doctor if you have phenylketonuria (PKU).

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Ondansetron may be used for the prevention or treatment of nausea and vomiting but it may cause constipation or a headache.

5. Tips

  • May be taken with or without food. Take oral tablets with a big glass of water.
  • Consider taking laxatives at the same time as ondansetron to prevent or relieve constipation.
  • When taking orally disintegrating ondansetron tablets, peel back the foil (do not attempt to push the tablet through the foil). Allow the tablet to dissolve in your mouth without chewing.
  • When taking the oral soluble strip film, use dry hands to remove the strip and place on your tongue. Leave to dissolve without chewing. Swallow several times after the strip dissolves. A glass of water may help with dissolution.
  • When taking ondansetron oral solution, measure the correct dosage with the dosing syringe provided.
  • Report any symptoms of serotonin syndrome to your doctor (symptoms include mental status changes [such as agitation, hallucinations, coma, delirium]), fast heart rate, dizziness, flushing, muscle tremor or rigidity and stomach symptoms [including nausea, vomiting, diarrhea]).
  • Report any other unusual side effects to your doctor.
  • If ondansetron makes you dizzy or drowsy, do not drive or operate machinery while you are affected and avoid alcohol.

6. Response and Effectiveness

The peak effect of ondansetron is seen within 1.5-2 hours. The onset of the effect is faster with the wafer form of ondansetron.

7. Interactions

Medicines that interact with ondansetron may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with ondansetron. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with ondansetron include:

  • antibiotics, such as clarithromycin or erythromycin
  • antidepressants, such as tricyclic antidepressants (eg, amitriptyline), monoamine oxidase inhibitors (eg, isocarboxazid, phenelzine, and tranylcypromine), venlafaxine, and SSRIs (eg, paroxetine, sertraline)
  • anticonvulsants, such as phenytoin, phenobarbital, or primidone
  • antipsychotics (such as butyrophenones, phenothiazines, or thioxanthenes) and atypical antipsychotics (eg, olanzapine, quetiapine, ziprasidone)
  • beta-blockers, such as atenolol or bisoprolol
  • lithium
  • medications that correct an irregular heartbeat, such as amiodarone or disopyramide
  • medications that induce or inhibit CYP2D6, CYP3A4, or CYP1A2 enzymes
  • opioids, such as oxycodone or morphine
  • other medications that affect serotonin, such as amphetamines, fentanyl, tramadol, triptans (eg, almotriptan, eletriptan, or sumatriptan), or St. John's Wort.

Note that this list is not all-inclusive and includes only common medications that may interact with ondansetron. You should refer to the prescribing information for ondansetron for a complete list of interactions.


Ondansetron. Revised 09/2019.

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use ondansetron only for the indication prescribed.

Copyright 1996-2021 Revision date: November 5, 2019.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.