First-generation antihistamines are medications used to relieve allergy symptoms, such as sneezing, runny nose, and itching. They work by blocking the action of histamine, a substance in the body that causes allergic reactions. Common examples include diphenhydramine and chlorpheniramine. These antihistamines can cause drowsiness because they cross the blood-brain barrier, affecting the central nervous system. While effective for allergies, their sedative effects make them less suitable for daytime use, especially for activities requiring alertness, such as driving or operating machinery.