Which statement about side effects of antihistamines is true?

Antihistamines are commonly used to alleviate symptoms caused by allergies, such as sneezing, itching, and runny nose, by blocking the action of histamine, a substance in the body that causes allergic symptoms. While they are effective in reducing these symptoms, one well-documented side effect of many antihistamines, particularly first-generation ones, is drowsiness. This occurs because these antihistamines can cross the blood-brain barrier and interact with histamine receptors in the central nervous system, leading to sedation.

The other statements do not accurately represent the properties of antihistamines. While they can relieve allergy symptoms, they do not prevent allergic reactions entirely. Weight gain is not a widely recognized side effect of antihistamines, though some patients might report changes in appetite or metabolism with certain medications. Additionally, antihistamines are not designed to improve respiratory functions; instead, they primarily target allergic symptoms. Therefore, the statement regarding drowsiness is accurate and highlights a significant consideration when using these medications.