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Antihistamines are classified by number – depending on what antihistamine receptor is blocked on cells – as H1, H2, and (more recently discovered) 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.
Older (first generation) antihistamines tend to 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 nerve cells communicate, allowing our brains to work properly. An antihistamine, therefore, can block the normal way our brains function (causing those side effects). Newer antihistamines, fortunately, do not get into the brain as readily, and so you may experience less side effects while taking them.