What is the mechanism of action of diphenhydramine?

• A. Analgesic
• B. Antibiotic
• C. Antihistamine
• D. Anticoagulant

Answer: (C)

Blocking histamine at H1 receptors is the mechanism of action of diphenhydramine. During allergic reactions, histamine binds to H1 receptors on tissues like nasal mucosa and blood vessels, causing symptoms such as itching, swelling, runny nose, and sometimes bronchoconstriction. Diphenhydramine acts as a competitive antagonist at these receptors, preventing histamine from binding and thereby reducing those symptoms. It’s a first-generation antihistamine, so it crosses the blood–brain barrier and can cause drowsiness and anticholinergic effects. It is not an analgesic, antibiotic, or anticoagulant.