April 28, 2023

Diphenhydramine (e.g. Benadryl) is an over-the-counter first generation antihistamine approved for allergy treatment by the United States Food and Drug Administration (FDA) in 1946. Diphenhydramine can also be used for its sedative and antiemetic effects. It acts as an antagonist for the histamine 1 (H1) receptor, reversing the effects of histamine in the body and reducing allergic reactions. Due to its ability to cross the blood brain barrier and antagonize the H1 receptors in the central nervous system (CNS), diphenhydramine can cause drowsiness and suppress the medullary cough center. Diphenhydramine also acts as a competitive antagonist for muscarine acetylcholine receptors and as a sodium channel blocker, and higher doses may lead to cardiotoxic effects. It is metabolized in the liver to desmethyldiphenhydramine and diphenylmethoxyacetic acid.