Diphenhydramine is classified as which drug class?

• A. Beta blocker
• B. Antihistamine
• C. Diuretic
• D. Proton pump inhibitor

Answer: (B)

Diphenhydramine is an antihistamine. It works by blocking histamine at H1 receptors, which reduces the allergic symptoms that histamine normally promotes—such as itching, swelling, runny nose, and sneezing. Because it’s a first-generation antihistamine, it can cross into the brain, often causing drowsiness and dry mouth due to anticholinergic effects. In EMS, it’s used as an adjunct for allergic reactions after epinephrine, not as a primary treatment for airway distress. It isn’t a beta blocker (which affects heart rate and blood pressure via adrenergic receptors), a diuretic (which increases urine output), or a proton pump inhibitor (which decreases gastric acid production).