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Ondansetron – Mechanism, Dosage and Side Effects

Ondansetron is an antiemetic medication used to prevent nausea and vomiting caused by chemotherapy, radiotherapy, or surgery. It works by blocking serotonin receptors in the gut and brain.

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Things worth knowing about "Ondansetron"

Ondansetron is an antiemetic medication used to prevent nausea and vomiting caused by chemotherapy, radiotherapy, or surgery. It works by blocking serotonin receptors in the gut and brain.

What is Ondansetron?

Ondansetron is a prescription medication belonging to the class of 5-HT3 receptor antagonists. It is used to prevent and treat nausea and vomiting associated with chemotherapy, radiation therapy, and surgical procedures. Ondansetron is one of the most widely used antiemetic drugs in modern clinical practice worldwide.

Mechanism of Action

Ondansetron selectively blocks 5-HT3 receptors (serotonin type 3 receptors) located in the gastrointestinal tract and the central nervous system. When chemotherapy or other triggers cause serotonin to be released from enterochromaffin cells in the gut, this serotonin normally activates 5-HT3 receptors, initiating the vomiting reflex. By blocking these receptors, ondansetron prevents the transmission of the nausea and vomiting signal to the brain.

Indications

Nausea and vomiting induced by chemotherapy (cytostatic-induced)
Nausea and vomiting caused by radiation therapy
Postoperative nausea and vomiting (PONV)
In some countries, off-label use for severe pregnancy-related nausea (hyperemesis gravidarum) – only under medical supervision

Dosage and Administration

Ondansetron is available in several formulations, including tablets, orally disintegrating tablets, injectable solutions, and suppositories. The dosage depends on the indication, the age of the patient, and body weight. A typical adult dose for chemotherapy-induced nausea is 8 mg taken orally before treatment, followed by additional doses every 8 to 12 hours. The exact dosing regimen should always be determined by a healthcare professional.

Special Patient Groups

Children: Ondansetron can be used in children above a certain age and weight threshold; dosing is adjusted individually.
Elderly patients: Dose adjustments are generally not required, but caution is advised.
Hepatic impairment: In patients with severe liver dysfunction, the total daily dose should not exceed 8 mg.

Side Effects

Ondansetron is generally well tolerated. The most commonly reported side effects include:

Headache
Constipation
Sensation of warmth or flushing
Fatigue

Less common but serious side effects include:

QT interval prolongation (a change in heart rhythm that can lead to dangerous arrhythmias)
Allergic reactions including anaphylaxis
Serotonin syndrome when combined with other serotonergic substances

Drug Interactions

Ondansetron may interact with several other medications. Particular caution is warranted with:

Drugs that also prolong the QT interval (e.g., certain antibiotics, antiarrhythmics)
Tramadol – reduced analgesic efficacy of tramadol is possible
Other serotonergic agents (e.g., SSRIs, MAO inhibitors) – risk of serotonin syndrome
Apomorphine – combination is contraindicated

Contraindications

Known hypersensitivity to ondansetron or related compounds
Congenital or acquired long QT syndrome
Concomitant use of apomorphine

Use During Pregnancy

The use of ondansetron during pregnancy, especially in the first trimester, remains a subject of ongoing debate. Some studies have suggested a slightly increased risk of certain birth defects, while other investigations have not confirmed this association. Use during pregnancy should therefore only occur after a careful risk-benefit assessment by a qualified healthcare provider.

References

European Medicines Agency (EMA): Summary of Product Characteristics for Ondansetron – www.ema.europa.eu
Brunton L.L., Hilal-Dandan R., Knollmann B.C.: Goodman & Gilman's The Pharmacological Basis of Therapeutics. 13th Edition. McGraw-Hill Education, 2017.
Rojas C., Slusher B.S.: Pharmacological mechanisms of 5-HT3 and tachykinin NK1 receptor antagonism to prevent chemotherapy-induced nausea and vomiting. European Journal of Pharmacology, 2012; 684(1-3):1-7. PubMed PMID: 22516113.

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