How Ondansetron Works: A Guide to Use and Safety

Nausea and vomiting are defense mechanisms prompted by the nervous system to rid the body of toxins. Though they serve a purpose, nausea and vomiting are extremely uncomfortable. Medications are available to help manage these conditions. One commonly prescribed option for nausea and vomiting is ondansetron (Zofran). 

Ondansetron is used to treat vomiting from chemotherapy, radiation therapy, or surgery. This article provides a comprehensive review of ondansetron's uses, dosage, side effects, and safety considerations.

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What Is Ondansetron?

Ondansetron (Zofran) was developed as an antiemetic (anti-vomiting) medication in the 1980s. It blocks a specific serotonin receptor called 5-HT3. It does this in two main ways:

• In the brain, it stops signals from the "chemoreceptor trigger zone," which detects things that can make you feel sick. 
• In the gastrointestinal tract, it blocks signals from the vagus nerve, which senses stomach problems. 

Together, these actions make ondansetron an effective option for managing nausea and vomiting in many cases.

FDA Approval and Uses

The FDA approved ondansetron in January 1991, and data over the years has supported its safety and efficacy when used as directed. However, due to the risks of heart rhythm changes, it is not approved in high-dose intravenous (IV) form. 

Ondansetron treats nausea and vomiting caused by anesthesia during surgery, chemotherapy, and radiation. It is sometimes used off-label to manage nausea and vomiting during pregnancy under medical supervision. Ondansetron is not effective at treating nausea from motion sickness. 

Approved Uses and Off-Label Applications

The following are approved and off-label applications:

Postoperative Nausea and Vomiting

Postoperative nausea and vomiting are common complaints after surgery and can be extremely uncomfortable for patients. Additionally, vomiting after surgery can increase pressure on surgical incisions and risk opening up sutures or glue. The risk of postoperative nausea and vomiting depends on many factors, including patient demographics, anesthesia type, and recovery. 

Ondansetron is one of the most common treatments for this condition and is often administered before or at the end of surgery. The medication is often administered as 8 mg in a dissolving tablet or 4 mg intravenously, depending on the patient’s needs.

Chemotherapy 

Chemotherapy can cause nausea and vomiting. Treating these side effects is a vital aspect of cancer care. 

• Clinical trials have shown that ondansetron has consistently demonstrated remarkable efficacy in alleviating both the delayed and acute phases of vomiting and nausea that are frequently triggered by chemotherapy regimens. 
• Radiation is another type of cancer treatment that can cause nausea and vomiting. Nausea associated with radiation can impact the efficacy of the treatments and reduce adherence to the radiation protocol. 
• Ondansetron can be given by mouth or intravenously to help reduce nausea and vomiting associated with radiation. 

Off-Label Applications

Nausea and vomiting during pregnancy are common in the early trimesters and can range from mild to severe. Hyperemesis gravidarum is a severe form of nausea and vomiting during pregnancy that may require hospitalization due to dehydration and weakness. 

• First-line treatments for nausea and vomiting during pregnancy are lifestyle interventions, vitamin B6, and antihistamines. 
• When these treatments are not effective, ondansetron may be considered under medical supervision.
• The dosage for this condition is 4 mg administered orally every 8 hours. 

Dosage and Administration

Standard Dosages

The dosing of ondansetron depends on weight, age, and indication. General dosing parameters are shown in Table 1:

Common Side Effects and Risks

Ondansetron is generally well tolerated. Common side effects have been reported in about 10% of users. These include:

• Constipation
• Dry Mouth
• Headache
• Fatigue
• Malaise

Less common adverse effects include:

• Drowsiness
• Itchy skin
• Reaction at the injection site
• Liver function test changes
• EKG changes

Rare But Serious Risks

Severe allergic reactions (anaphylaxis) are possible. If signs of an allergic reaction appear, such as a skin rash or difficulty breathing, seek immediate medical attention. People with allergies to ondansetron or any of its ingredients should not take the medication.  

Precautions and Interactions

Ondansetron is not suitable for everyone:

• It is contraindicated (not recommended) in patients taking apomorphine. 
• For patients with a history of abnormal heart rhythms, additional cardiac monitoring may be necessary while they are taking ondansetron. 
• This medication may be considered during pregnancy under medical supervision if other treatments have not been effective. Decisions about ondansetron use during pregnancy require a thorough risk versus benefit assessment to address potential concerns about fetal safety. This medication appears to be safe during breastfeeding.

FAQs and Practical Insights

Can ondansetron be used long-term?

Ondansetron is typically used short term to relieve nausea and vomiting. There is limited research on its safety for long-term use. 

What to do if you miss a dose?

If you miss a dose of ondansetron and you feel nauseous, take the missed pill as soon as you can. 

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Key Takeaways

• Ondansetron (Zofran) is an antiemetic medication developed in the 1980s to treat nausea and vomiting caused by chemotherapy, radiation therapy, surgery, and off-label, pregnancy-related sickness.
• It blocks 5-HT3 serotonin receptors in the brain and gastrointestinal tract, preventing signals that trigger nausea and vomiting.
• Ondansetron is FDA-approved for managing nausea and vomiting associated with surgery, chemotherapy, and radiation. It is sometimes used off-label for pregnancy-related nausea under medical supervision but is not considered effective for motion sickness.
• Common side effects include constipation, dry mouth, headache, and fatigue. Serious adverse effects, like heart rhythm changes or allergic reactions, are rare but possible.
• Patients with heart rhythm problems or allergies to ondansetron should avoid it. Its use during pregnancy requires careful risk assessment, but it is generally safe during breastfeeding.
• Ondansetron is typically used short term and should be taken as prescribed. Always consult your doctor to tailor the treatment to your needs and ensure it fits into a comprehensive care plan.
• Consulting a healthcare provider ensures the safe and effective use of ondansetron, especially for patients with special circumstances.
• If you experience nausea and vomiting that affect your daily life, consult your healthcare provider to determine if ondansetron may be an appropriate option for you.