What are first-generation antihistamines primarily used for?

First-generation antihistamines are primarily used to treat allergic reactions due to their ability to block the action of histamine at H1 receptors. Histamine is a chemical involved in the body's immune response, particularly in allergic reactions, leading to symptoms such as itching, sneezing, runny nose, and hives. By inhibiting the effects of histamine, first-generation antihistamines effectively alleviate these symptoms, making them a common choice for conditions such as allergic rhinitis and allergic conjunctivitis.

These medications are known for their sedative properties, which can be an added benefit in some cases, such as when allergies also interfere with sleep. The older formulations of these antihistamines are often less selective and can cross the blood-brain barrier, resulting in sedation. This sedative effect distinguishes them from newer, second-generation antihistamines, which tend to have fewer central nervous system side effects and are preferred for long-term use in managing allergies.

Other options, such as treating infections, lowering blood pressure, or reducing cholesterol levels, do not pertain to the mechanism of action or therapeutic uses of first-generation antihistamines, which are not indicated for these conditions.