Final answer:
First-generation antihistamines are older drugs that can cause drowsiness, while second-generation antihistamines reduce allergic symptoms with less impact on the central nervous system. Antihistamines work by opposing the activity of histamine receptors and are classified based on the histamine receptor they act upon, with two main classes being H₁ and H₂-antihistamines.
Step-by-step explanation:
First-generation and second-generation medications are terms used to categorize drugs based on their pharmacological characteristics and side effects. Using antihistamines as an example, first-generation antihistamines were among the earliest developed drugs in this class and can cross the blood-brain barrier. This leads to effects on the central nervous system, such as drowsiness. Second-generation antihistamines are newer and have molecular structures that are less likely to cross the blood-brain barrier, thereby reducing central nervous system side effects like sedation.
Antihistamines work by blocking the action of histamine, a substance in the body that promotes allergic symptoms. The first-generation antihistamines are often used to treat symptoms such as nasal congestion, sneezing, or hives triggered by allergies. However, they also have sedative effects, making them useful for conditions such as insomnia. On the other hand, second-generation antihistamines have the advantage of less sedation, making them favorable for individuals who require relief from allergy symptoms without the drowsiness.
Antihistamines differ from many other drugs in that they are specifically antagonists to histamine receptors. Whereas H₁-antihistamines focus on receptors responsible for allergic reactions and some central nervous system functions, H₂-antihistamines are targeted towards gastric acid conditions, acting at different receptor sites in the body. Thus, their classification is based on both the type of histamine receptor they act upon and their generation.