antiemetic
An antiemetic calms the body's vomiting alarm. Vomiting is a reflex coordinated by the brainstem, set off by triggers as varied as motion, toxins, certain drugs, or a churning gut — and antiemetics work by blocking the chemical signals that ring that alarm.
Different antiemetics target different receptors in this reflex. Serotonin 5-HT3 antagonists (ondansetron) blunt signals from the gut and the brain's chemoreceptor trigger zone and are central to chemotherapy and post-operative nausea. Dopamine D2 antagonists (metoclopramide, domperidone, prochlorperazine) work at the trigger zone and also speed gastric emptying. Antihistamines and antimuscarinics (cyclizine, hyoscine) suppress motion-related nausea, and neurokinin NK1 antagonists (aprepitant) help with delayed chemotherapy sickness.
Matching the drug to the cause is the key skill: 5-HT3 antagonists do little for motion sickness, while antihistamines do little for chemotherapy-induced vomiting. Side effects follow the receptor too — dopamine blockers can cause restlessness or movement disorders, and 5-HT3 antagonists can cause constipation and QT-interval prolongation.
Before chemotherapy a patient receives ondansetron, a 5-HT3 antagonist, to prevent the intense nausea the cytotoxic drugs would otherwise cause.
Ondansetron blocks 5-HT3 receptors to control chemotherapy-induced vomiting.
Choosing an antiemetic means asking why the patient is vomiting; the most effective drug depends on whether the trigger is the gut, the inner ear, the brain's trigger zone, or a drug like chemotherapy.