Ondansetron: Mechanism of Action & Contraindications

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  • 0:04 Ondansetron
  • 0:43 Mechanism of Action
  • 1:38 Adverse Drug Reactions
  • 2:30 Contraindications
  • 3:00 Pregnancy & Lactation
  • 3:42 Lesson Summary
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Lesson Transcript
Instructor: Lisa Cauthen

Lisa is a Registered Nurse with a 14 years of experience and a Masters Degree in Nursing Education. She has certifications in CPN, ACLS, PALS, and NRP.

Ondansetron is a drug given to prevent and treat nausea and vomiting. This lesson will review its mechanism of action, adverse drug reactions, and contraindications.


Ondansetron, commonly referred to by its brand name Zofran, is a drug that was first developed and studied in the 1980s and is now widely approved for and used in the prevention and treatment of nausea and vomiting associated with chemotherapy, radiation, and surgery. It's currently available in oral and injectable forms.

Susan Smith recently underwent an uncomplicated surgery to remove her appendix. She was prescribed ondansetron for her post-operative nausea and vomiting. Susan prides herself on being a knowledgeable person, and as she's unfamiliar with ondansetron, she has some questions about this medication.

Mechanism of Action

Susan's first question is how ondansetron works. She doesn't like taking medications without understanding what action they're performing in her body. After learning that the vomiting center in the human brain is rich in chemoreceptors, including dopamine, histamine, serotonin, and acetylcholine, Susan understands that many of the drugs developed to minimize nausea and vomiting, including ondansetron, are designed to block the uptake of these chemicals.

Ondansetron and the other members of the -setron drug family are serotonin antagonists. This means these drugs block the serotonin receptors in the chemoreceptor trigger zone (CTZ) within the medulla oblongata. The CTZ communicates with the vomiting center to initiate vomiting. By blocking the serotonin receptors, there's less serotonin that enters the CTZ, which decreases communication with the vomiting center. Ultimately, the patient experiences reduced nausea and vomiting.

Adverse Drug Reactions

Susan's next question is about side effects she will experience while taking ondansetron. It's explained to her that not all patients experience side effects while taking this medication, but there is a possibility she'll experience one. Side effects are also known as adverse drug reactions.

Adverse drug reactions (ADR) for ondansetron can include headache, dizziness, constipation, and/or diarrhea. Some patients also experience drowsiness after taking ondansetron. Some medical providers feel that this side effect is not caused by the drug itself, but rather the patient is feeling better and able to rest. While taking ondansetron, some patients may have increased liver enzyme levels. These levels usually return to normal when the patient stops taking the medication. Rarely, ondansetron can cause cardiac arrhythmias, or abnormal heart rhythms, such as long QT syndrome.


Susan wants to know if there are any circumstances when she (or anyone) should not take ondansetron. Ondansetron, as with all medications, is contraindicated in patients with an allergy or hypersensitivity to the drug. Because of its ability to increase liver enzyme levels, ondansetron should be used cautiously in patients with impaired liver function. In addition, because of the risk for arrhythmias, patients with underlying cardiac disorders, such as long QT syndrome, should be monitored carefully if given ondansetron.

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