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ACAAI > Patients & Public > Resources > Ask the Allergist

Effects of Oral Antihistamines on Allergy Symptoms

Q.  Will oral antihistamines take care of all my allergy symptoms?  

A.  Oral antihistamines relieve many of the allergy symptoms by blocking histamine, the chemical "culprit" that causes many symptoms. Antihistamines are classified by number - depending on what antihistamine receptor is blocked on cells - as H1 and H2 (and newer ones discovered are H3 and H4).  The H1 and H4 receptors are associated with capillaries and nerves, such as in the nose or skin, and H2 receptors are found in the lining of the stomach.  Even the best antihistamines do not offer anything in terms of permanent relief, nor do they relieve nasal congestion. Rather, they temporarily relieve symptoms such as: sneezing, itching, nasal drainage and hives.

Regarding side effects, older (first generation) antihistamines have drawbacks that include being short-acting (measured in hours), and causing drowsiness, making it difficult to concentrate.  Newer (second generation) antihistamines, many of which are now over the counter, are not as likely to cause this problem. Most of us think of histamine negatively, since we associate it with allergic reactions. However, histamine is actually a chemical which helps never cells communicate, thus allowing our brains to work properly. An "antihistamine," therefore, may block the normal way our brains function, just as older antihistamines are likely to do. Newer antihistamines fortunately do not get into the brain as readily.