Antiemetics

Classification

5-HT3 antagonists: ondansetron, granisetron, palonosetron. D2 antagonists: metoclopramide, domperidone, antipsychotics(low-dose). NK1 antagonists: aprepitant, fosaprepitant. H1/anticholinergics: diphenhydramine, promethazine, hyoscine. Corticosteroid: dexamethasone(adjunct). Cannabinoids: dronabinol, nabilone.

Vomiting Pathway — 4 Input Sources (organizing framework)

CTZ(area postrema, OUTSIDE BBB, exposed to circulating chemicals) — rich D2+5-HT3+NK1 → chemo/drug-induced nausea.
Vestibular system — rich H1+muscarinic → motion sickness/vertigo.
GI tract(vagal afferents) — peripheral 5-HT3(enterochromaffin cell serotonin, chemo/radiation mucosal damage) → CTZ+vomiting centre directly.
Higher cortical centres — ANTICIPATORY nausea(learned, anxiety-related, repeated chemo cycles) bypasses peripheral triggers entirely → responds to BENZODIAZEPINES(anxiolysis), not receptor-targeted antiemetics.

Mechanism, Matched to Indication

5-HT3 antagonists: block serotonin PERIPHERALLY(vagal, enterochromaffin) + CENTRALLY(CTZ) → BOTH mechanisms → 1st-line CINV(chemotherapy-induced N&V) + PONV.

D2 antagonists: block CTZ D2(same mechanism as antipsychotics, here exploiting CTZ not mesolimbic effect). Metoclopramide: ALSO separate PROKINETIC action(5-HT4 agonism+D2 antagonism in GI tract → ↑gastric emptying+LES tone) → gastroparesis/GERD use beyond pure antiemetic. Domperidone: doesn’t cross BBB appreciably(unlike metoclopramide) → same CTZ benefit, MUCH less central EPS risk — specific PK distinction, same core mechanism.

NK1 antagonists(aprepitant): block substance P at NK1(CTZ+GI) — substance P = DELAYED-phase CINV mediator(>24h post-chemo, when 5-HT3 mechanism subsided) → add-on to 5-HT3 antagonist+dexamethasone for highly emetogenic regimens, NOT monotherapy.

H1/anticholinergics: block histamine/muscarinic at VESTIBULAR NUCLEI → specifically preferred for MOTION SICKNESS(vestibular pathway other classes don’t address).

Dexamethasone: mechanism not fully understood — genuinely effective CINV adjunct(+5-HT3 ± NK1 antagonist), thought = direct central effect + ↓treatment-related inflammation contributing to nausea.

Cannabinoids: CB1 receptors at CTZ/vomiting centre — reserved for CINV REFRACTORY to standard regimens(less favourable SE profile limits 1st-line use).

Adverse Effects

5-HT3 antagonists: headache+constipation(5-HT3 also mediates normal GI motility, peripheral block slows transit) = most common. QT prolongation(class-wide caution, esp ondansetron/dolasetron).

Metoclopramide: EPS(same D2-blockade mechanism as antipsychotics — ACUTE DYSTONIA specific risk even with SHORT-TERM antiemetic use, not just chronic antipsychotic therapy) + hyperprolactinaemia(tuberoinfundibular). → domperidone often preferred where appropriate(minimal CNS penetration); metoclopramide use should be TIME-LIMITED not chronic.

Antihistamines/anticholinergics: standard antimuscarinic/antihistaminic profile(sedation, dry mouth, blurred vision, urinary retention) — recurring cluster from multiple prior topics, not new.

NK1 antagonists: well tolerated generally; fatigue + CYP3A4 interaction(substrate AND moderate inhibitor/inducer — complicates co-administration with chemo agents/other CYP3A4 drugs, relevant given oncology context).

Cannabinoids: dysphoria/euphoria, sedation, dependence potential → reserved role despite genuine efficacy.

Clinical Relevance

4-input-source framework = genuine MATCHING exercise, not trial-and-error: motion sickness(vestibular) responds to antihistamines/anticholinergics specifically because it doesn’t meaningfully involve CTZ/GI-vagal pathways. CINV’s BIPHASIC mechanism(acute serotonin-driven, delayed substance-P-driven) = exactly why modern regimens combine 5-HT3 antagonist+NK1 antagonist+dexamethasone rather than relying on ONE agent — 3 different mechanisms addressing 3 different, temporally-distinct triggers in the SAME clinical scenario.

Classification (by Receptor Target)

5-HT3 receptor antagonists: ondansetron, granisetron, palonosetron
D2 receptor antagonists: metoclopramide, domperidone, and the antipsychotics (prochlorperazine, haloperidol at low dose) when used for this purpose
NK1 (neurokinin-1) receptor antagonists: aprepitant, fosaprepitant
H1 receptor antagonists/anticholinergics: diphenhydramine, promethazine, hyoscine (scopolamine)
Corticosteroids: dexamethasone (an adjunct antiemetic, mechanism distinct from and incompletely understood relative to the receptor-targeted classes)
Cannabinoids: dronabinol, nabilone

The Vomiting Pathway — Organizing Framework

Nausea and vomiting are coordinated by the vomiting centre in the medulla, which integrates input from four principal sources, each triggered by a different clinical situation and each best matched by a different antiemetic class — this input-source framework is the organizing principle for the whole topic:

Chemoreceptor trigger zone (CTZ): located in the area postrema, outside the blood-brain barrier (directly exposed to circulating chemicals/drugs/toxins in the blood and CSF) — rich in D2 dopamine receptors, 5-HT3 serotonin receptors, and NK1 receptors, making it the primary target for chemotherapy-induced and drug-induced nausea.
Vestibular system: signals via the vestibular nuclei, rich in H1 histamine and muscarinic acetylcholine receptors — the primary pathway for motion sickness and vertigo-related nausea.
GI tract (vagal afferents): peripheral serotonin release from enterochromaffin cells (particularly relevant to chemotherapy- and radiation-induced GI mucosal damage) signals via vagal 5-HT3 receptors to both the CTZ and directly to the vomiting centre.
Higher cortical centres: anticipatory nausea (a learned, anxiety-related response, particularly recognized in patients undergoing repeated chemotherapy cycles) bypasses the peripheral triggers above entirely, working through cortical/limbic pathways instead — the specific reason this particular cause of nausea responds better to benzodiazepines (via anxiolysis) than to any of the receptor-targeted antiemetics that address the other three input sources.

Mechanism of Action, Matched to Indication

5-HT3 antagonists (ondansetron): block serotonin’s action both peripherally (vagal afferents from the GI tract, particularly relevant to enterochromaffin-cell serotonin release triggered by chemotherapy/radiation mucosal damage) and centrally at the CTZ — the combination of peripheral and central blockade is why this class is specifically first-line for chemotherapy-induced nausea and vomiting (CINV), which involves both mechanisms simultaneously, and also effective for postoperative nausea/vomiting (PONV).

D2 antagonists (metoclopramide, domperidone): block dopamine receptors at the CTZ (directly analogous to typical antipsychotics’ D2-blocking action described under Antipsychotic and Antimanic Drugs, here exploited for its CTZ effect rather than its mesolimbic antipsychotic effect) — metoclopramide additionally has a genuinely separate prokinetic action (5-HT4 receptor agonism plus D2 antagonism in the GI tract itself, increasing gastric emptying and lower oesophageal sphincter tone), making it specifically useful for gastroparesis and GERD-related symptoms beyond its pure antiemetic action, a dual-mechanism drug worth distinguishing clearly from a “pure” antiemetic. Domperidone doesn’t cross the blood-brain barrier appreciably (unlike metoclopramide), giving it the CTZ-targeted antiemetic/prokinetic benefit with substantially less central extrapyramidal risk (below) — a specific, examined pharmacokinetic distinction between two drugs sharing the same core D2-antagonist mechanism.

NK1 antagonists (aprepitant): block substance P’s action at the NK1 receptor, both in the CTZ and in the GI tract — substance P is a specific, delayed-phase mediator of chemotherapy-induced emesis (particularly the delayed phase occurring more than 24 hours after chemotherapy, when 5-HT3-mediated mechanisms have largely subsided) — aprepitant’s specific value is in this delayed phase, used as an add-on to a 5-HT3 antagonist plus dexamethasone for highly emetogenic chemotherapy regimens rather than as monotherapy.

H1/anticholinergics (promethazine, hyoscine): block histamine and/or muscarinic receptors at the vestibular nuclei — the specific mechanistic reason this class, rather than any of the receptor-targeted classes above, is preferred for motion sickness (a vestibular-pathway problem the other classes don’t specifically address).

Corticosteroids (dexamethasone): mechanism as an antiemetic is not fully understood, but it is a genuinely effective, evidence-based adjunct in CINV regimens (combined with a 5-HT3 antagonist ± NK1 antagonist), thought to involve both a direct central antiemetic effect and reduction of the peritumoral/treatment-related inflammation that can itself contribute to nausea.

Cannabinoids (dronabinol, nabilone): act on CB1 receptors in the CTZ/vomiting centre — reserved generally for chemotherapy-induced nausea refractory to standard first-line regimens, given a less favourable overall side-effect profile (below) limiting first-line use.

Adverse Effects

5-HT3 antagonists: headache and constipation (5-HT3 receptors also mediate normal GI motility, so blocking them peripherally slows transit) are the most common effects; QT prolongation is a specific, examined class-wide caution (more prominent with ondansetron/dolasetron than with the others), requiring caution with other QT-prolonging drugs and in patients with relevant cardiac risk factors.

Metoclopramide: extrapyramidal symptoms (the same D2-blockade-driven mechanism already covered fully under Antipsychotic and Antimanic Drugs — acute dystonia in particular is a specific, recognized risk even with short-term antiemetic use, not just with chronic antipsychotic therapy) and hyperprolactinaemia (same tuberoinfundibular-pathway mechanism) — these central D2-blockade effects are specifically why domperidone (minimal CNS penetration) is often preferred where available and appropriate, and why metoclopramide use is generally recommended to be time-limited rather than chronic.

Antihistamines/anticholinergics: the standard antimuscarinic/antihistaminic profile (sedation, dry mouth, blurred vision, urinary retention) already described in multiple prior topics — worth recognizing as the same recurring adverse-effect cluster rather than a new fact set specific to this indication.

NK1 antagonists: generally well tolerated; fatigue and, notably, a specific CYP3A4 interaction (aprepitant is both a substrate and moderate inhibitor/inducer, complicating co-administration with numerous chemotherapy agents and other CYP3A4-dependent drugs — an examined, clinically relevant interaction given the oncology context these drugs are typically used in).

Cannabinoids: dysphoria/euphoria, sedation, and dependence potential — the reason for their reserved, not-first-line role despite genuine antiemetic efficacy.

Clinical Relevance

The four-input-source framework (CTZ, vestibular, GI/vagal, cortical) is what makes antiemetic drug selection a genuine matching exercise rather than trial-and-error — motion sickness (vestibular) responds to antihistamines/anticholinergics specifically because it doesn’t meaningfully involve the CTZ or GI-vagal pathways the other classes target, and CINV’s specific biphasic mechanism (acute serotonin-driven, delayed substance-P-driven) is exactly why modern chemotherapy antiemetic regimens combine a 5-HT3 antagonist with an NK1 antagonist and dexamethasone rather than relying on any single agent — three genuinely different mechanisms addressing three genuinely different, temporally distinct triggers within the same clinical scenario.

Vomiting centre with its four input sources (CTZ, vestibular, GI/vagal, cortical) each mapped to the antiemetic class that targets it.
CINV biphasic timeline (acute serotonin-driven vs delayed substance-P-driven) with each drug’s window of effectiveness marked.