Almost nothing about the antiemetics has to be memorised drug by drug. There is one coordinating centre in the medulla; there are five afferent routes into it; each route carries a transmitter, and each of the large drug classes blocks exactly one of those transmitters. Learn the map and the drug list becomes a lookup table — which class covers motion sickness and which does not, why one agent is never enough for a high-risk patient, why a drug that blocks dopamine in the trigger zone also blocks it in the striatum and the pituitary, and why the answer to a patient who has vomited despite ondansetron is never more ondansetron.
Orientation
What this module covers, and how to work through it
This module covers one line of the MMed syllabus — the physiology of vomiting and the antiemetic classes — which is one line of syllabus and three sittings of reading. It runs from the reflex outwards: what the vomiting centre is and what feeds it, then the drugs that block each afferent in turn, then the agents that do not fit that scheme and the argument for combining them.
Lesson 1 settles the material every other lesson applies. The vomiting centre is not a nucleus but a collection of effector neurones; the chemoreceptor trigger zone sits functionally outside the blood-brain barrier, which is why a circulating drug can make a patient vomit without ever entering the brain; and five separate afferent routes converge on the centre, which is the whole reason no single antiemetic covers every cause. The lesson then turns that into a clinical decision: which risk factors are real, which have been disproven, how the Apfel score converts to a predicted risk, and how much a first, second and third drug actually buy.
Lesson 2 is the bulk of the pharmacology and the part the written paper keeps returning to: the 5-HT3 antagonists, the dopamine antagonists in their three structural groups, the H1 antagonists and the antimuscarinics. It is organised so that the side effects are derived rather than listed — block D2 in the chemoreceptor trigger zone and you have also blocked it in the nigrostriatal pathway and at the pituitary, and the extrapyramidal effects and the hyperprolactinaemia follow from that, not from a table.
Lesson 3 takes the agents that sit outside that scheme. Dexamethasone works and nobody has settled why. The NK1 antagonists block a transmitter none of the other classes touch, which is exactly why they add to a regimen instead of duplicating it. Then the adjuncts, and the arithmetic that makes a multimodal regimen worth building.
Prerequisites for the module as a whole
These carry material this module builds on directly and does not repeat. Work through them first if they are not already secure:
- The blood-brain barrier: what the barrier is made of and what crosses it. The chemoreceptor trigger zone is defined by sitting outside it, and half of this module’s pharmacology — domperidone against metoclopramide, glycopyrrolate against hyoscine — is an argument about crossing it.
- Ionisation and pKa: why a permanently charged quaternary amine behaves differently from an uncharged tertiary one. Lesson 2 turns that into the difference between glycopyrrolate and hyoscine.
- Synaptic transmission and receptors: ligand-gated channels against G-protein-coupled receptors. The 5-HT3 receptor is the former and the NK1 receptor is the latter, and this module says so without re-deriving either.
Syllabus
MMed syllabus mapping
| Syllabus area | Lessons | What is covered |
|---|---|---|
| General pharmacology — the physiology of vomiting | 1 | The vomiting centre and its composition; the chemoreceptor trigger zone and the blood-brain barrier; the five afferent routes and the transmitter each carries; the motor sequence of vomiting, and its distinction from retching and regurgitation; the receptor-to-class map |
| General pharmacology — postoperative nausea and vomiting | 1 | Definition, early against late, incidence with and without risk factors; patient, surgical and anaesthetic risk factors, including the disproven ones; the Apfel simplified score and the separate postdischarge score; a risk-stratified prophylaxis algorithm; reducing the afferent load by technique rather than by drug |
| General pharmacology — the antiemetic classes | 2–3 | 5-HT3 antagonists in full; dopamine antagonists as phenothiazines, butyrophenones and benzamides, with the extrapyramidal, prolactin and QT consequences derived from where D2 sits; H1 antagonists and their anticholinergic contribution; the antimuscarinics and the central anticholinergic syndrome; dexamethasone and its unsettled mechanism; NK1 antagonists; benzodiazepines, cannabinoids and non-pharmacological adjuncts; combination and rescue therapy |
The histamine, antihistamines and phenothiazines objective is not settled by this module, and deliberately carries no link to it. The phenothiazines are taught here in full — the three structural subgroups, the receptor profile, the effects and the kinetics — and the H1 antagonists are taught in their antiemetic indication in lesson 2. Histamine as a transmitter is not taught here at all, and neither is any antihistamine use outside emesis, so pointing that objective here would tick a line of syllabus a reader has met two-thirds of. Ephedrine and the sympathomimetics likewise belong to the adrenergic objective; ephedrine appears in one catalogued question paired with ondansetron, and that does not make it an antiemetic.
The pathway
All 3 lessons, in order
Part I
The reflex, and the decision
What coordinates vomiting, what feeds it, and how to decide whether this particular patient should be given a drug at all.
- The reflex, and who is worth treating
One medullary centre, five afferent routes into it, four receptors on those routes — and the risk arithmetic that decides whether a given patient is given anything at all.
Part II
The drugs
The receptor antagonists first, because they are most of the pharmacology; then the agents that sit outside that scheme, and the case for using several at once.
- The four receptors, and the drugs that block them
5-HT3, dopamine, histamine and muscarinic: fourteen drugs whose entire side-effect profile can be read off the receptor they block and the tissue that receptor sits in.
- The steroid, the pitants, and why two beats twice one
A drug that works by a mechanism nobody has settled, a class that blocks a transmitter none of the others touch, the adjuncts worth naming — and the arithmetic of combining them.
Objectives
The full objective wording
- Vomiting and prophylaxisThe physiology of vomiting, and deciding who needs prophylaxis
- The receptor antagonistsThe receptor antagonists: 5-HT3, dopamine, histamine and muscarinic
- Steroids, NK1 and combinationsDexamethasone, NK1 antagonists, adjuncts and combination therapy