Antiemetics

Vomiting is coordinated by a brainstem "vomiting center" that receives input from four main sources: the chemoreceptor trigger zone (CTZ), which sits outside the blood-brain barrier and samples blood and cerebrospinal fluid for toxins and drugs; the vestibular apparatus, which signals motion and position; vagal afferents from the gut, which respond to irritation, distension, and chemotherapy-damaged mucosa; and the cortex, which processes anxiety, sensory triggers, and anticipatory learning. Each pathway relies on different neurotransmitters — serotonin (5-HT3) and substance P (NK1) dominate gut and CTZ signaling, dopamine (D2) is prominent in the CTZ, histamine (H1) and acetylcholine (muscarinic) drive vestibular signaling, and cannabinoid receptors modulate multiple sites. This receptor map explains why no single antiemetic works for every cause.

5-HT3 Receptor Antagonists

Ondansetron, granisetron, and palonosetron block serotonin at 5-HT3 receptors on gut vagal afferents and in the CTZ, making them first-line for chemotherapy-induced and postoperative nausea and vomiting. They are generally well tolerated; common effects include headache and constipation. All can prolong the QT interval, so they warrant caution in patients with electrolyte imbalances, existing QT prolongation, or other QT-prolonging drugs. Palonosetron has a longer half-life and higher receptor affinity, making it useful for delayed nausea.

Dopamine (D2) Antagonists

Metoclopramide blocks D2 receptors in the CTZ and also has prokinetic action in the gut, useful for nausea tied to slow gastric emptying. It carries a boxed warning for tardive dyskinesia with prolonged or high-dose use, so therapy duration is limited. Prochlorperazine, a phenothiazine, is used for moderate nausea and vertigo but can cause sedation and extrapyramidal symptoms. Promethazine works well but carries a severe risk of tissue injury, including necrosis, if it extravasates or is given intra-arterially; it is also markedly sedating, raising fall risk. Droperidol is effective but requires QT monitoring due to a known risk of dose-dependent prolongation and rare arrhythmia.

Antihistamines and Anticholinergics

Dimenhydrinate and meclizine block H1 receptors involved in vestibular signaling, making them the standard choice for motion sickness. Scopolamine, delivered as a transdermal patch, blocks muscarinic receptors in the vestibular pathway and is favored for prolonged exposure such as sea travel. Sedation and anticholinergic effects (dry mouth, blurred vision, urinary retention) are common across this class, adding fall risk in older adults.

NK1 Receptor Antagonists

Aprepitant and its prodrug fosaprepitant block substance P at neurokinin-1 receptors, which is especially important for delayed chemotherapy-induced nausea occurring more than 24 hours after treatment. They are typically added to a 5-HT3 antagonist and dexamethasone rather than used alone.

Adjuncts and Other Options

Dexamethasone, a corticosteroid, has antiemetic effects through mechanisms not fully understood and is a mainstay adjunct for chemotherapy-related nausea, often combined with a 5-HT3 antagonist and an NK1 antagonist for highly emetogenic regimens. Benzodiazepines such as lorazepam are used for anticipatory nausea, which is a learned, anxiety-driven response to prior chemotherapy experiences rather than a direct pharmacologic trigger. Olanzapine, an atypical antipsychotic, blocks multiple receptors (dopamine, serotonin, histamine, and others), giving it broad-spectrum activity useful in refractory or breakthrough nausea. Cannabinoids act on cannabinoid receptors involved in the modulation of nausea signaling and are used when other classes fail or are poorly tolerated.

Practical Framework

Rational prescribing starts with identifying the cause: motion or vertigo points to antihistamines or scopolamine; chemotherapy points to 5-HT3 antagonists plus dexamethasone, adding an NK1 antagonist for highly emetogenic regimens; postoperative nausea often responds to a 5-HT3 antagonist or low-dose dopamine antagonist; anticipatory nausea calls for a benzodiazepine alongside behavioral strategies. High emetogenic risk generally calls for combining agents from different receptor classes rather than escalating one drug, since blocking multiple pathways simultaneously improves control and limits dose-related toxicity. Nausea in pregnancy is typically managed starting with doxylamine-pyridoxine before escalating to other agents, reserving stronger antiemetics for refractory cases. Nursing considerations include monitoring QT intervals and electrolytes with 5-HT3 antagonists and droperidol, protecting peripheral IV sites and recognizing early signs of extravasation with promethazine, and assessing fall risk in patients receiving sedating antihistamines, anticholinergics, or dopamine antagonists.