Antihistamines

Antihistamines compete with H1 histamine receptors, thereby preventing the effects of histamine.
The Antihistamines are able to prevent or reduce increased capillary permeability. This action decreases edema (swelling) , itching, and bronchospasms.

Antihistamines are divided into first generation and second generation. Those that fall into first generation bind to central and peripheral receptors and can cause central nervous system depression (e.g., sedation) or stimulation. Those of the second generation cause less sedation.

They are most effective with nasal allergies, and when used at the beginning of symptoms. They should not be regarded as a sole treatment with asthma. Antihistaminics are considered palliative, providing symptomatic relief, and are not a cure. The relief of symptoms is obtained only while one of these drugs is being taken.

The antihistaminics are well absorbed and most are metabolized by the liver and excreted in the urine.