Antihistamines are drugs that block the action of histamine (a compound released in allergic inflammatory reactions) at the H1 receptor sites, responsible for immediate hypersensitivity reactions such as sneezing and itching.

Members of this class of drugs may also be used for their side effects, including sedation and antiemesis .

Antihistamines provide their primary action by blocking histamine H1 at the receptor site. They have no effect on rate ofhistamine release, nor do they inactivate histamine.

By inhibiting the activity of histamine, they can reduce capillaryfragility, which produces the erythema or redness, associated with allergic reactions.

They will also reducehistamineinduced secretions, including excessive tears and salivation.

Additional effects vary with the individual drugused. Several of the older drugs, called first-generation antihistamines, bind non-selectively to H1 receptors in the centralnervous system as well as to peripheral receptors and can produce sedation, inhibition of nausea and vomiting, andreduction of motion sickness.

Drugs that combat the histamine released during an allergic reaction by blocking the action of the histamine on the tissue.

Antihistamines do not stop the formation of histamine nor do they stop the conflict between the IgE and antigen.

Therefore, antihistamines do not stop the allergic reaction but protect tissues from some of its effects. Antihistamines frequently cause mouth dryness and sleepiness.