Which antiemetic is commonly given as a 4 mg IV dose in the ED?

• A. Metoclopramide
• B. Dimenhydrinate
• C. Promethazine
• D. Ondansetron

Answer: (D)

The main idea here is choosing an antiemetic that is routinely given as a standard 4 mg IV dose in the emergency department. Ondansetron fits this role because it is a selective 5-HT3 receptor antagonist that acts quickly when given intravenously, producing rapid relief of nausea and vomiting within minutes and lasting for a few hours. It has a favorable safety profile with minimal sedation and a low risk of extrapyramidal symptoms, which makes it a preferred first-line option in many ED scenarios.

Metoclopramide is commonly dosed higher (often around 10 mg IV) and carries a higher risk of movement disorders due to dopamine blockade. Dimenhydrinate and promethazine are antihistamines with anticholinergic effects and sedating properties; their standard IV doses are higher (and their side-effect profiles can be less desirable in the ED). Those differences in dosing and tolerability make them less likely to be the routinely used 4 mg IV antiemetic in many ED protocols, compared with ondansetron.