What is the maximum dose of diphenhydramine?

• A. 25 mg
• B. 50 mg
• C. 75 mg
• D. 100 mg

Answer: (B)

Diphenhydramine is given in EMS as an adjunct for allergic reactions, and the dose is limited per administration to balance benefit with potential anticholinergic and sedative side effects. In typical paramedic protocols, the maximum single dose is 50 mg, given by IM or IV. Doses of 50 mg provide the needed antihistamine effect without a disproportionate jump in adverse effects; increasing beyond 50 mg raises risks such as excessive drowsiness, confusion, urinary retention, tachycardia, and hypotension, with limited added therapeutic benefit in the acute prehospital setting. Therefore, 50 mg per dose is the appropriate maximum. Smaller doses (like 25 mg) may be used for sensitivity or pediatrics, but 50 mg remains the per-dose ceiling.