Which of the following is an example of a first generation antihistamine?

Diphenhydramine is recognized as a first-generation antihistamine due to its chemical structure and pharmacological properties. First-generation antihistamines are known for their ability to cross the blood-brain barrier, leading to central nervous system effects such as sedation. Diphenhydramine is commonly used for the treatment of allergic reactions, motion sickness, and as a sleep aid, demonstrating its versatility among first-generation agents.

On the other hand, the other options represent second-generation antihistamines, which are less likely to cross the blood-brain barrier. Cetirizine, Loratadine, and Fexofenadine are designed to minimize sedation and central nervous system effects, making them preferable for long-term use in managing allergic symptoms while reducing unwanted drowsiness. This distinction highlights the characteristic differences between the generations of antihistamines, emphasizing why Diphenhydramine stands out as the correct example of a first-generation antihistamine.