Antiemetics are medications used to prevent or treat nausea and vomiting (emesis). They are widely used in clinical settings such as postoperative recovery, chemotherapy, motion sickness, and gastrointestinal illnesses.
One of the most commonly prescribed antiemetics is Ondansetron (Zofran), a selective serotonin receptor antagonist. This article covers antiemetics with a focus on Ondansetron, explaining their mechanism of action, clinical uses, adverse effects, interactions, and contraindications.
Introduction to Antiemetics
Class:
- Pharmacologic (P): Selective Serotonin (5-HT3) Receptor Antagonists
- Therapeutic (T): Antiemetics
Common Examples of Antiemetics:
- Ondansetron (Zofran) — 5-HT3 antagonist
- Promethazine — Antihistamine
- Metoclopramide — Dopamine antagonist
- Scopolamine — Anticholinergic
- Dimenhydrinate — Antihistamine (motion sickness)
Ondansetron is administered via:
- PO (oral)
- IV
- IM
Mechanism of Action (MOA)
Ondansetron works by blocking 5-HT3 serotonin receptors in two major places:
1. Chemoreceptor Trigger Zone (CTZ) in the brain
- This area detects toxins and triggers vomiting.
- Blocking serotonin here → prevents nausea signals.
2. Vagus Nerve Terminals in the GI tract
- GI irritation releases serotonin → stimulates vomiting.
- Blocking serotonin → reduces gut-induced nausea.
Result:
- ↓ Nausea
- ↓ Vomiting
- Relief in postoperative patients, chemotherapy-induced nausea, and GI causes
Mnemonic for MOA:
“Zofran stops the serotonin signal → stops the vomit reflex.”
Therapeutic Uses of Antiemetics
Antiemetics help manage nausea from multiple causes:
| Condition | How Antiemetics Help |
|---|---|
| Motion sickness | Reduce inner ear-triggered nausea |
| Gastroenteritis | Calm vagus nerve activity |
| Chemotherapy-induced N/V | Prevent CTZ activation |
| Postoperative nausea | Standard prophylaxis in PACU |
| Pregnancy-related nausea | Some antiemetics used with caution |
| Medication-induced nausea | Opioids, antibiotics, general anesthetics |
Ondansetron is especially effective for postoperative and chemotherapy-related nausea.
Adverse Effects (Mnemonic: GAG)
To remember Ondansetron’s major side effects, use the mnemonic “GAG” — fitting because it prevents gagging and vomiting.
| Letter | Side Effect | Description |
|---|---|---|
| G | Groggy & Dizzy | Mild CNS effects like headache, dizziness |
| A | Arrhythmias | Risk of QT prolongation → ventricular arrhythmias |
| G | GI Discomfort | Nausea, vomiting, diarrhea can still occur |
Other Possible Side Effects:
- Constipation
- Fatigue
- Fever
- Elevated liver enzymes (rare)
Ondansetron can prolong the QT interval, especially in high IV doses.
Drug Interactions
1. Opioids
- Combined use → increased risk of hypotension
- Monitor BP closely during pain management
2. Antidepressants (SSRIs, SNRIs, TCAs)
- Increased risk of serotonin syndrome
- Greater risk of ventricular arrhythmias with QT-prolonging agents
3. Antiarrhythmic Drugs (amiodarone, sotalol)
- Additive QT prolongation
Contraindications
Avoid or use cautiously in:
- Prolonged QT interval (congenital or acquired)
- Hepatic impairment (Ondansetron metabolized in liver)
- Electrolyte disturbances (hypokalemia, hypomagnesemia → worsens QT)
- Concurrent QT-prolonging medications
High-risk patients should have cardiac monitoring when receiving IV Ondansetron.
Nursing Considerations
Before Administration
- Assess nausea triggers and severity
- Check baseline ECG if patient has cardiac history
- Review medication profile for QT-prolonging drugs
During Administration
- Administer IV Ondansetron slowly to avoid risk of arrhythmias
- Monitor for relief of nausea and improvement in oral intake
- Assess for dizziness or headache
After Administration
- Monitor bowel movements (risk of constipation or diarrhea)
- Continue hydration assessment
- Check liver function in long-term therapy or hepatic disease
Patient Education
- Report irregular heartbeat, dizziness, or fainting
- Safe to take with food or without
- Avoid alcohol (worsens dizziness)
- Notify provider if nausea persists despite treatment
Types of Antiemetic Drugs (Overview)
While Ondansetron is the prototype in this image, antiemetics belong to multiple pharmacologic classes:
| Drug Class | Example | Mechanism |
|---|---|---|
| 5-HT3 Antagonists | Ondansetron | Block serotonin in CTZ & gut |
| Dopamine Antagonists | Metoclopramide | Block dopamine → increase GI motility |
| H1 Antihistamines | Dimenhydrinate | Block vestibular system → motion sickness |
| Anticholinergics | Scopolamine | Block muscarinic receptors → motion sickness |
| NK1 Antagonists | Aprepitant | Block substance P in vomiting center |
PPIs or H2 blockers may also help reduce nausea linked to acid disorders.
Summary Table
| Parameter | Details |
|---|---|
| Drug Class | Antiemetic; 5-HT3 receptor antagonist |
| Example | Ondansetron (Zofran) |
| Mechanism | Blocks serotonin at CTZ & vagus nerve |
| Uses | Prevent nausea & vomiting (post-op, chemo, GI) |
| Adverse Effects (GAG) | Grogginess, Arrhythmias, GI upset |
| Interactions | Opioids, antidepressants, antiarrhythmics |
| Contraindications | QT prolongation, hepatic impairment |
| Routes | PO, IV, IM |
FAQs About Antiemetics
Q1. Are antiemetics safe for children?
Yes, but dosing must be adjusted. Ondansetron is commonly used for pediatric dehydration-related nausea.
Q2. Can pregnant women use Ondansetron?
It may be used cautiously when benefits outweigh risks. Alternatives like Diclegis are often used first.
Q3. How long does Ondansetron take to work?
Usually within 30 minutes for oral doses; faster with IV.
Q4. Can Ondansetron cause constipation?
Yes, mild constipation or diarrhea is common.
Q5. Which antiemetic is best for motion sickness?
Antihistamines like dimenhydrinate or anticholinergics like scopolamine.
