Diphenhydramine HCl is an ethanolamine-class 1st generation histamine H1 receptor antagonist and a common anticholinergic. The anti-allergic, antiemetic and antitussive effects of Diphenhydramine are well established and Diphenhydramine is the most commonly used active ingredient in antihistamines for allergy medication. Diphenhydramine is a major component of many cold preparation products and hypnotics.