What is an antihistamine used to treat?

An antihistamine is primarily used to treat allergic reactions and symptoms associated with allergies, such as hay fever, hives, and allergic conjunctivitis. These medications work by blocking the action of histamine, a substance in the body that causes allergic symptoms. When allergens enter the body, they trigger the release of histamine, which can lead to symptoms such as sneezing, itching, runny nose, and swelling. By inhibiting histamine, antihistamines help alleviate these uncomfortable symptoms, effectively managing an allergy attack. In contrast, the other options—headaches, diabetes, and heart conditions—do not fall under the scope of antihistamine treatment, as these conditions would require different types of medications or interventions tailored to their specific physiological mechanisms.