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Updated August 28, 2008


Antihistamines are a class of medications that block the action of histamine, a substance that is released by cells in your body in response to an allergic reaction. The release of histamine is responsible for many allergy symptoms, such as sneezing, runny nose and itching. Some antihistamines are also used for their side effects, including sedation and prevention of nausea and vomiting.

Antihistamines are available by prescription and over the counter.


Examples of prescription antihistamines include Allegra (fexofenadine), Astelin Nasal Spray (azelastine) and Zyrtec (cetirizine).

Examples of over-the-counter antihistamines include Claritin (loratadine), Benadryl (diphenhydramine) and Chlor-Trimeton (chlorpheniramine).

Diphenhydramine, the active ingredient in Benadryl, causes sedation and is used in many over-the-counter sleep aids.

To learn more about your antihistamine, use Drugs A to Z.

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