Pharmacotherapy Newsletter

Volume 1, Issue 15, 04/27/2009

Question

How does the antihistamine, diphenhydramine (Allerdryl®; Benadryl®; Tylenol PM®; Unisom®) actually cause xerostomia (dry mouth)?

Answer

The antihistamine, diphenhydramine (Allerdryl®; Benadryl®; Tylenol PM®; Unisom®) has been used for many years for a variety of conditions that include such things as allergies, sleep aid, and motion sickness.^1,2  Most clinicians and even patients know very well that older antihistamines (such as diphenhydramine) cause xerostomia (dry mouth).^1,2  In fact, xerostomia is one of many side effects associated with the use of diphenhydramine that are collectively called "anticholinergic" side effects which also includes such things as drowsiness, blurry vision, constipation and urinary retention.  This means that older antihistamines, like diphenhydramine, block some of the actions mediated by the cholinergic system and thus are described as having anticholinergic effects in addition to their histamine (H1) antagonist activity.¹ Why is diphenhydramine more likely to cause anticholinergic side effects like xerostomia?