About Diphenhydramine First Generation H1 Antagonist ,Ethanolamine derivative, Antihistamine, antiemetic, antivertigo,sedative, and hypnotic. Mechanism of Action of Diphenhydramine Antiallergic action: It produces its antihistamine action by competitively blocking H1 receptors. It binds to the H1 receptors present in the smooth muscles of the gastrointestinal tract, bronchi, large blood vessels and uterus and inhibit Histamine induced allergic symptoms. They do not inhibit the release of Histamine. Antivertigo and antiemetic actions: Central antimuscarinic actions of antihistamines are responsible for these effects of Diphenhydramine. Parkinsonism: Acetyl choline receptor antagonists are used for the treatment of Parkinsonism. Diphenhydramine reduces the unbalanced cholinergic activity in striatum of parkinsonian patients. Antitussive action: Diphenhydramine suppresses the cough reflex by a direct effect on the cough centre. Migraine: Diphenhydramine is used in migraine due