Which statement about first-generation H1 antihistamines is true?

Which statement about first-generation H1 antihistamines is true?

• A. They are non-sedating antihistamines.
• B. They block histamine H2 receptors.
• C. They are sedating antihistamines that antagonize H1 receptors and can cause sedation.
• D. They are monoclonal antibodies against histamine.

Answer: (C)

First-generation H1 antihistamines work by blocking histamine at H1 receptors, which helps relieve allergic symptoms like sneezing and itching. Their lipophilicity allows them to cross the blood–brain barrier, leading to central nervous system effects such as sedation. This combination—antagonizing H1 receptors and causing sedation—is what defines them as sedating antihistamines. They do not block H2 receptors (those affect gastric acid secretion), nor are they monoclonal antibodies against histamine. In contrast, second-generation antihistamines are less sedating precisely because they penetrate the brain less readily.

First-generation H1 antihistamines work by blocking histamine at H1 receptors, which helps relieve allergic symptoms like sneezing and itching. Their lipophilicity allows them to cross the blood–brain barrier, leading to central nervous system effects such as sedation. This combination—antagonizing H1 receptors and causing sedation—is what defines them as sedating antihistamines. They do not block H2 receptors (those affect gastric acid secretion), nor are they monoclonal antibodies against histamine. In contrast, second-generation antihistamines are less sedating precisely because they penetrate the brain less readily.