Antihistaminic Drugs

a group of substances capable of suppressing the action of free histamine, a biologically active substance occurring in cells and tissues. Antihistaminic drugs relieve spasms of the smooth muscles of the bronchi and intestines caused by histamine, decrease capillary permeability, inhibit the development of edema caused by histamine, and relieve allergic reactions.

The mechanism of antihistaminic-drug action is not entirely clear. It is thought that the antihistaminic activity of these drugs is explained by their displacement of histamine from the biochemical tissue systems and by blockage of the receptors of the cells of the smooth muscles and glands. Some data show that antihistaminic drugs increase the activity of histaminase, an enzyme that promotes the decomposition of histamine. Along with antihistaminic action, many antihistaminic drugs have a depressive effect on the central nervous system; cause drowsiness; reinforce the effects of narcotics, somnifacients, analgesics, and local anesthetics; lower body temperature; act as anti-inflammatories and prevent nausea and vomiting in motion sickness; relieve vomiting in pregnancy; and produce other effects. These properties are most strongly evident in diprazin and etisine, to a somewhat lesser degree in dimedrol, and to a considerably weaker degree in suprastin. A few antihistaminic drugs—for example, diazoline—exert no depressive effect on the central nervous system.

Antihistaminic drugs are used mainly in allergic diseases, in the initial stage of bronchial asthma, in Ménière’s disease, and similar ailments. A number of these drugs (dimedrol, diprazin) may be used to prevent motion sickness as well as to treat parkinsonism, chorea, and excessive vomiting in pregnancy and to relieve symptoms of radiation sickness and the like.

Antihistaminic drugs are taken internally according to a physician’s prescription, more rarely in subcutaneous, intramuscular, or intravenous injections. They are usually well-tolerated by patients; however, in some persons they cause dizziness. Prolonged use sometimes causes changes in the blood (leukopenia). Because of their depressive effect on the central nervous system, the majority of antihistaminic drugs should not be taken while performing work requiring rapid mental and physical reactions—for example, driving an automobile.