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Definition of Antihistamine


Updated November 06, 2012

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.


A type of compound or drug present in many cold and allergy medications that may decrease the effects of histamine, a chemical present in allergic reactions. These reactions may be triggered by various substances called allergens. Histamines are active in inflammation, and work by increasing the leakiness of small blood vessels called capillaries, causing the release of water from the blood vessels into nearby tissues, which become swollen. As a result, symptoms such as nasal congestion and runny nose occur. Drugs in the antihistamine class block the action of histamines, which can open the airway and narrow blood vessels. They can also act to decrease the effects on nerves within the nose that may cause sneezing or itching. Antihistamines are therefore effective in the treatment of hay fever or allergies.

One of an antihistamine's most common side effects is sedation, making these medications attractive as over-the-counter sleeping pills to treat acute insomnia. Common examples include diphenhydramine (sold as Benadryl or included in medicines such as Tylenol PM or Advil PM) and doxylamine (sold as Unisom or Nyquil). Unfortunately, they may also commonly result in residual daytime sleepiness, or a "hangover" effect. These medications are not the best sleeping pill option and should not be used for longer than 2 weeks.

I take an antihistamine medication if I am exposed to cats because I am allergic.
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