Which of the following is a first-generation H1 blocker?

• A. Diphenhydramine
• B. Cetirizine
• C. Loratadine
• D. Fexofenadine

Answer: (A)

First-generation H1 blockers cross the blood–brain barrier easily, leading to sedation and often more anticholinergic effects. Diphenhydramine is a classic example of this older group, so its CNS-drowsiness profile and anticholinergic side effects are typical of first-generation antihistamines. The other drugs listed—cetirizine, loratadine, and fexofenadine—are second-generation antihistamines designed to minimize CNS entry, so they cause much less drowsiness and have fewer anticholinergic effects. Because of that, they’re considered non-sedating or less sedating options and are favored for daytime use. Thus, diphenhydramine is the first-generation H1 blocker among these options, with the distinctive sedating profile of the older class. Use with caution in the elderly and when activities require alertness.