synthetic drug used to counteract the histamine reaction, as in allergies (allergy). Chlorpheniramine, introduced into medicine in 1951, is administered orally or by intravenous, intramuscular, or subcutaneous injection in the form of chlorpheniramine maleate. It is effective in controlling the symptoms of hay fever, acute skin reactions (such as hives), and contact dermatitis (such as from poison ivy). The most common side effect is drowsiness, although dryness of the mouth, difficulty in urinating, and vision problems also may occur.