A drug may be classified by the chemical type of the active ingredient or by the way it is used to treat a particular condition. Each drug can be classified into one or more drug classes. Antihistamines are drugs that inhibit the action of histamine in the body by blocking the receptors of histamine. There are two types of histamine receptors H1 and H2. When H1 receptors are stimulated by histamine it may produce allergic reactions such as itching, hay fever and rash or hives.