Description a histamine h1 antagonist used in allergic reactions, hay fever, rhinitis, urticaria, and asthma. it has also been used in veterinary applications. one of the most widely used of the classical antihistaminics, it generally causes less drowsiness and sedation than promethazine.indication :for the treatment of rhinitis, urticaria, allergy, common cold, asthma and hay fever.mechanism of action :chlorpheniramine binds to the histamine h1 receptor. this blocks the action of endogenous histamine, which subsequently leads to temporary relief of the negative symptoms brought on by histamine.