Zonalon is a drug that belongs to a drug class known as topical antihistamines. Zonalon has an active component known as doxepin HCL. Zonalon is generally used to treat various skin conditions and to relieve persistent itching. The active ingredient, doxepin, is also known as a tricyclic antidepressant. Zonalon can treat skin diseases like eczema and pruritus because it has antipruritic and antihistamine properties that stop the allergic reactions from happening.