First-generation H1 antihistamines are the older antihistamines, developed from the 1940s onward, that pass readily across the blood-brain barrier. Because histamine helps maintain wakefulness in the brain, blocking it centrally produces sedation. Most also have some anticholinergic activity.
Hydroxyzine, diphenhydramine, chlorpheniramine, and promethazine are commonly cited members of this group. Their sedating quality is sometimes an unwanted side effect and sometimes the reason a clinician selects them.