Antihistamines work by targeting receptor sites in the airways, blood vessels, skin and nose to prevent or block the release of histamine, a chemical released at the receptor site when an allergic reaction is occurring. Histamine is naturally produced by the body to fight off foreign agents, however, in some individuals the receptor sites become over stimulated and release an excess of histamine. This excess histamine can irritate the nose and eyes causing hay fever symptoms such as sneezing and itchy and watery eyes and nose. An antihistamine's mechanism of action is to coat the allergen receptors which will prevent the binding of histamine with special receptor sites in the nose and throat. Coating these receptor sites and blocking the action of histamines in the body will reduce and prevent swelling and inflammation, the cause of allergy symptoms. Antihistamines can only be used to treat histamine induced allergic conditions such as rhinitis, conjunctivitis, angioedema, anaphylactic reactions, sedation or nausea. An antihistamine will work to stop the allergic reaction and thus the symptoms. If taken before the onset of symptoms antihistamines can actually prevent allergy symptoms. Generally the antihistamine drug will begin to work about 20 minutes after administration and reach full effectiveness in about 1 to 2 hours.

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