Ondansetron (Zofran)

Drug Class

Mechanism of Action

1. Peripheral - on vagal afferent nerve fibers lining the GI tract (enterochromaffin cells release serotonin during chemotherapy/radiation/surgery, which normally triggers the vomiting reflex via these vagal fibers)
2. Central - in the chemoreceptor trigger zone (CTZ) of the brain

• Katzung's Basic and Clinical Pharmacology, 16th Ed.
• Lippincott Illustrated Reviews: Pharmacology

Indications

• Chemotherapy-induced nausea and vomiting (CINV) - highly or moderately emetogenic regimens (first-line)
• Radiation-induced nausea and vomiting
• Postoperative nausea and vomiting (PONV)
• Undifferentiated nausea/vomiting in the ED (first-line)
• IBS-diarrhea (off-label, some evidence)
• Gastroenteritis with vomiting (especially in children)

Dosage

• Hepatic impairment (severe): Do NOT exceed 8 mg/day total (only 5-HT3 antagonist requiring hepatic dose adjustment)
• Children: IV ondansetron used in infants as young as 1 month; oral studies generally include children ≥6 months
• IV dose: 0.45 mg/kg total daily, max 32 mg/day (older guideline; FDA later capped single IV dose at 16 mg due to QT risk)

Half-Life & Pharmacokinetics

Side Effects

Common

• Headache (most common)
• Constipation (slows colonic transit - class effect of 5-HT3 blockade)
• Diarrhea
• Dizziness
• Fatigue

Serious / Clinically Important

Drug Interactions & Precautions

• Avoid or use with caution in patients with:
  • Congenital long QT syndrome
  • Congestive heart failure
  • Bradyarrhythmias
  • Concurrent QT-prolonging medications
  • Hypokalemia or hypomagnesemia (correct electrolytes before giving)
• Consider an ECG before administration in patients with cardiac history

Tintinalli's Emergency Medicine, Lippincott Pharmacology, Rosen's Emergency Medicine

Key Advantages Over Older Antiemetics

• No dopamine blockade - so no extrapyramidal effects (no acute dystonia, no akathisia)
• No sedation (unlike promethazine)
• Effective across all grades of emetogenic chemotherapy
• Can be given as a single pre-treatment dose before chemo or surgery

In Pregnancy

Ondansetron - Simple Explanation

What is it?

How does it work? (Mechanism)

When your stomach is upset (from chemo, surgery, or illness), your gut releases a chemical called serotonin. This serotonin sends a signal to your brain saying "vomit now!"

• In the gut (stops the signal from starting)
• In the brain (stops the signal from arriving)

Dose (How much to give?)

Half-Life (How long does it stay in body?)

• 3 to 6 hours - meaning after 3-6 hours, half the drug is gone from your blood
• Effect lasts about 4 to 9 hours
• Broken down by the liver, removed by kidneys (urine)

Side Effects (What can go wrong?)

Common (not dangerous):

• 🤕 Headache - most common
• 😴 Dizziness
• 💩 Constipation - because it slows the gut
• 😩 Fatigue (tiredness)

Serious (watch out!):

• ❤️ QT prolongation - it can affect the heart's electrical rhythm at high doses. Can cause dangerous heart rhythms
• Avoid in people with heart problems or low potassium/magnesium

Why is it better than older anti-vomiting drugs?

One Line Summary

Ondansetron blocks serotonin receptors in the gut and brain to stop vomiting. Give 4-8 mg every 8 hours. Lasts 4-9 hours. Main risks are headache and heart rhythm issues at high doses.

Serotonin - Simple Explanation

What is Serotonin?

Where is it made?

• 90% in the GUT (intestines) - yes, most serotonin is in your stomach, not brain!
• 8% in blood platelets
• 2% in the brain

What does Serotonin do?

The "Happy Chemical"

• High serotonin = good mood, calm, happy
• Low serotonin = depression, anxiety, poor sleep

Serotonin and Vomiting (linking back to Ondansetron)

Gut is irritated
      ↓
Gut releases SEROTONIN
      ↓
Serotonin hits 5-HT3 receptors on nerves
      ↓
Signal travels to brain's "vomiting center"
      ↓
You VOMIT 🤮

Serotonin Receptors

• 5-HT3 - the vomiting receptor (blocked by ondansetron)
• 5-HT1 - mood and migraine (targeted by triptans)
• 5-HT4 - gut movement

What happens with TOO MUCH serotonin?

• Agitation, confusion
• Fast heart rate
• High temperature (fever)
• Muscle twitching
• Can be life-threatening

Quick Summary

Serotonin is a natural chemical in your body that controls mood, digestion, sleep, and vomiting. It's the "happy chemical" in the brain, but also the "vomit trigger" in the gut. Too little = depression. Too much = serotonin syndrome.

Serotonin & How It Causes Vomiting

First - What is Serotonin?

Where is Serotonin Stored?

90% of serotonin is stored in your GUT - in special cells called Enterochromaffin cells (EC cells)

Now - Step by Step: How Serotonin Causes Vomiting

Imagine you ate something poisonous or received chemotherapy:

STEP 1 - The Gut Gets Irritated

Poison / Chemo / Bad food enters the gut
            ↓
Gut wall gets irritated and damaged

STEP 2 - EC Cells Release Serotonin

Enterochromaffin cells detect the irritation
            ↓
They RELEASE serotonin into the gut wall

STEP 3 - Serotonin Hits the Nerve

Serotonin travels to nearby nerve endings
            ↓
It attaches to 5-HT3 receptors on the VAGUS NERVE

STEP 4 - Signal Travels to the Brain

Vagus nerve carries the signal upward
            ↓
Signal reaches the VOMITING CENTER in the brain
(called the "Area Postrema" / Chemoreceptor Trigger Zone)

STEP 5 - Brain Commands Vomiting

Vomiting center receives the alarm
            ↓
Brain sends command to stomach, diaphragm, mouth
            ↓
YOU VOMIT 🤮

Full Picture in One Diagram

BAD SUBSTANCE enters gut
        ↓
EC cells release SEROTONIN
        ↓
Serotonin attaches to 5-HT3 receptor
        ↓
VAGUS NERVE carries signal to brain
        ↓
VOMITING CENTER activated
        ↓
VOMITING happens

How Ondansetron Stops This

BAD SUBSTANCE enters gut
        ↓
EC cells release SEROTONIN
        ↓
🚫 ONDANSETRON BLOCKS 5-HT3 receptor
        ↓
Signal CANNOT reach vagus nerve
        ↓
Vomiting center is NOT activated
        ↓
NO VOMITING ✅

Simple Real-Life Analogy

One Line to Remember

When the gut is irritated, EC cells release serotonin → serotonin activates the vagus nerve → vagus nerve tells the brain → brain causes vomiting. Ondansetron blocks serotonin so this chain never completes.