AIM STEP 10
Student Memory Support
Nausea and Vomiting: Antiemetic Pharmacology
4th Year MBBS • GIT Module • High-yield memory reinforcement and last-minute revision. :contentReference[oaicite:0]{index=0}
1. High-Yield Flashcards
Tap each question to reveal the answer.
Which receptor pathway is especially important in acute chemotherapy-induced vomiting?
Serotonin acting at 5-HT3 receptors on vagal afferents and central emetic pathways.
Name three important 5-HT3 receptor antagonists.
Ondansetron, granisetron and palonosetron.
Which adverse effects should be remembered with 5-HT3 antagonists?
Headache, constipation, dizziness and possible QT-interval prolongation.
Which receptors dominate the vestibular pathway in motion sickness?
Histamine H1 and muscarinic M1 receptors.
Which antimuscarinic drug is particularly useful for preventing motion sickness?
Scopolamine (hyoscine).
Why do first-generation H1 antihistamines cause sedation?
They cross the blood-brain barrier and block central H1 receptors.
What are the two major pharmacological actions of metoclopramide?
D2 receptor blockade for antiemesis and 5-HT4 receptor stimulation for prokinetic activity.
Which important motor adverse effects can occur with metoclopramide?
Acute dystonia, parkinsonian symptoms and tardive dyskinesia.
Why can metoclopramide interfere with levodopa therapy?
Its central D2 blockade opposes the dopaminergic effect required in Parkinson disease.
Why does domperidone cause fewer extrapyramidal effects than metoclopramide?
Domperidone penetrates the blood-brain barrier poorly.
Which important cardiac caution is associated with domperidone?
QT prolongation with risk of cardiac arrhythmia.
How do neuroleptic antiemetics such as prochlorperazine reduce vomiting?
They block dopamine D2 receptors in the chemoreceptor trigger zone.
What is the main antiemetic role of benzodiazepines?
They reduce anticipatory nausea by decreasing anxiety and conditioned responses through GABA-A enhancement.
What is the antiemetic role of dexamethasone?
It acts as an adjunct that enhances antiemetic control, especially in chemotherapy-associated and postoperative vomiting.
Which classic drug combination is used for nausea and vomiting of pregnancy?
Doxylamine with pyridoxine.
2. Mnemonics
Mnemonic Title: Major Antiemetic Receptor Targets
SHDMN
Meaning: Serotonin 5-HT3 • Histamine H1 • Dopamine D2 • Muscarinic M1 • Neurokinin NK1.
Mnemonic Title: Metoclopramide Key Adverse Effects
EPD
Meaning: Extrapyramidal effects • Prolactin increase • Diarrhea from increased gastrointestinal motility.
Mnemonic Title: Chemotherapy Vomiting Phases
SNA
Meaning: Serotonin for acute emesis • NK1/substance P for delayed emesis • Anxiety for anticipatory nausea.
3. Memory Tables
Metoclopramide vs Domperidone
| Feature | Metoclopramide | Domperidone |
|---|---|---|
| D2 blockade | Central + peripheral | Predominantly peripheral |
| CNS penetration | Significant | Poor |
| EPS | Important risk | Much less frequent |
| Prolactin | May increase | May increase |
| Main caution | EPS / tardive dyskinesia | QT prolongation |
Cause of Vomiting vs Useful Drug Target
| Clinical Setting | Key Pathway | Useful Drug |
|---|---|---|
| Motion sickness | H1 / M1 | H1 antihistamine / scopolamine |
| Acute CINV | 5-HT3 | Ondansetron-class drug |
| Delayed CINV | NK1 / substance P | Aprepitant |
| Anticipatory nausea | Higher-centre anxiety response | Lorazepam adjunct |
| Gastroparesis-related nausea | D2 + impaired motility | Metoclopramide |
4. Rapid Revision Points — Last-Minute Revision
Must Remember:
- 5-HT3 antagonists block both peripheral vagal and central serotonin-mediated emetic signalling.
- Ondansetron-class drugs are especially useful for acute chemotherapy-induced vomiting.
- H1 antihistamines and scopolamine are useful when vestibular pathways dominate.
- Scopolamine may cause dry mouth, blurred vision, urinary retention and confusion.
- Metoclopramide combines D2 antagonism with 5-HT4-mediated prokinetic activity.
- Metoclopramide may cause dystonia, parkinsonism, hyperprolactinemia and tardive dyskinesia.
- Domperidone causes fewer central motor effects because CNS penetration is poor.
- Aprepitant targets NK1-mediated delayed chemotherapy vomiting.
- Benzodiazepines are adjuncts for anticipatory nausea, not primary serotonin antagonists.
- Dexamethasone strengthens combination antiemetic therapy despite an incompletely defined mechanism.
Common KMU Trap: Do not treat all vomiting as the same pharmacological problem. Drug selection depends on the dominant pathway: vestibular H1/M1, serotonin 5-HT3, dopamine D2, NK1/substance P or anticipatory higher-centre mechanisms.
5. Clinical Memory Hooks
Sea travel with nausea → vestibular H1/M1 signalling → think H1 antihistamine or scopolamine.
Gastroparesis with nausea → impaired gastric emptying → metoclopramide gives antiemetic plus prokinetic benefit.
Vomiting within hours of chemotherapy → serotonin/5-HT3 pathway → ondansetron-class drug.
Nausea before the next chemotherapy cycle begins → conditioned anxiety response → lorazepam adjunct.
6. Starred High-Yield Exam Points
- ⭐ Acute chemotherapy-induced vomiting: serotonin release → 5-HT3 activation → 5-HT3 antagonist.
- ⭐ Motion sickness: vestibular H1/M1 pathway → H1 antihistamine or scopolamine.
- ⭐ Metoclopramide: D2 antagonist + 5-HT4 agonist → antiemetic plus prokinetic effect.
- ⭐ Metoclopramide can worsen parkinsonian symptoms and oppose levodopa through central D2 blockade.
- ⭐ Domperidone: fewer extrapyramidal effects because of poor CNS penetration, but QT prolongation is an important caution.
- ⭐ Delayed chemotherapy vomiting: substance P/NK1 pathway → aprepitant.
- ⭐ Morning sickness: doxylamine with pyridoxine is a classic pharmacological combination.