A family of drugs used to treat allergic conditions, such as itching and hives. The drugs work by blocking the action of histamine, a chemical that is released during an allergic reaction. Examples of antihistamines include diphenhydramine, promethazine, terfenadine, and chlorpheniramine.

Without treatment, histamine dilates small blood vessels, causing redness and swelling; antihistamines block this effect, while preventing the irritation of nerve fibers that would otherwise cause itching. Antihistamines are the most effective treatment for hives.

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