First vs. Second Generation Antihistamines
Hydroxyzine is a first-generation antihistamine, which means it crosses the blood–brain barrier easily and exerts effects on the central nervous system (which explains it sedative and anxiolytic effects). Newer, second-generation antihistamines like cetirizine (tradename: Reactine) are less able to cross the blood–brain barrier. Thus, they are marketed as non-drowsy and have less side effects such as memory impairment.