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What is a common side effect of diphenhydramine?

A drowsiness

Diphenhydramine is the active ingredient in Benadryl and several other over-the-counter cough and cold medications and pain relievers. Diphenhydramine is an antihistamine associated with drowsiness and other "slowing" anticholinergic side effects, such as dry eyes, dry mouth, constipation, and urinary retention.