Which of the following antihistamines is classified as a 2nd-generation antihistamine?

Cetirizine is correctly classified as a 2nd-generation antihistamine. These medications are designed to alleviate allergy symptoms such as sneezing, itching, and runny nose while minimizing sedation as a side effect, which is a common issue with first-generation antihistamines.

Second-generation antihistamines, like cetirizine, have a more selective action on peripheral H1 histamine receptors and are less likely to cross the blood-brain barrier compared to first-generation agents. This makes them less sedating and allows for ease of use throughout the day without significant drowsiness.

In contrast, the other agents listed, such as diphenhydramine, chlorpheniramine, and promethazine, belong to the first generation of antihistamines. These older antihistamines often result in sedation due to their ability to penetrate the central nervous system and block histamine receptors in the brain. Therefore, cetirizine's classification as a 2nd-generation antihistamine highlights its improved safety profile concerning sedation and effective management of allergic symptoms.