Which of the following side effects is most commonly associated with first-generation antihistamines like diphenhydramine?

A. Drowsiness: Diphenhydramine readily crosses the blood–brain barrier and blocks central H1 receptors. Central nervous system depression commonly results in sedation and impaired alertness. This effect is more pronounced with first-generation antihistamines compared to newer agents. B. Diarrhea: Diphenhydramine has anticholinergic properties that reduce gastrointestinal motility. Decreased motility is more likely to cause constipation rather than diarrhea. Diarrhea is not a common adverse effect of first-generation antihistamines. C. Bradycardia: First-generation antihistamines do not typically slow the heart rate. Anticholinergic effects are more likely to cause mild tachycardia due to vagal inhibition. Bradycardia is not a characteristic effect of diphenhydramine. D. Restlessness: Sedation is more common than agitation in adults taking diphenhydramine. Restlessness or paradoxical excitation may occur, particularly in children or older adults, but it is not the most common effect. Drowsiness remains the predominant side effect.