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Define Antihistamines

Antihistamines are a class of medications used to treat the millions who suffer from allergies and their symptoms. Although some antihistamines are available by prescription only, a large number of the drugs are available in over-the-counter (OTC) formulations. Antihistamines are frequently used in combination with other medications, such as pain relievers and decongestants, to provide relief from ailments with multiple symptoms.

  1. Histamines

    • Histamines are proteins released by the body's immune system in reaction to certain outside substances and phenomena. These can range from dust and pollen that have been inhaled to some skin irritants, such as poison ivy or the bite of a mosquito. Once released, histamines usually bind to receptors on cells in the part of the body that first reacted to the outside substance, be it the skin or upper respiratory complex. The histamines set off a complex series of physiological reactions, including inflammation, itching and redness.

    Antihistamines to the Rescue

    • Antihistamines can block the effects of histamines when taken well in advance of exposure to known allergens, according to Discovery Health's Allergy Center. They do this by coating cell receptors, thus making it impossible for histamines to bind themselves to those receptors. The effectiveness of antihistamines after exposure to an allergen is generally not nearly as thorough as that attained when using them preventatively.

    Types

    • There are two types of antihistamines readily available in pharmacies, according to FamilyDoctor.org. These are the so-called first-generation and second-generation antihistamines. While first-generation antihistamines generally produce the side effect of drowsiness, this is not true of most of the second-generation group. Products with first-generation antihistamines are more likely to be available in OTC products than those in the second generation. Popular first-generation antihistamines include brompheniramine, chlorpheniramine, dimenhydrinate, diphenhydramine and doxylamine. Second-generation drugs include cetirizine, fexofenadine, desloratadine and loratadine.

    Side Effects and Interactions

    • Antihistamines produce very few side effects in healthy adults, except for the drowsiness characteristic of the first generation of these medications. However, FamilyDoctor.org cautions that they may interact negatively with such other medications as tranquilizers, anti-anxiety drugs, sleeping pills, muscle relaxants and high blood pressure medications. If you are taking any of these drugs on a regular basis, check with your doctor before taking antihistamines.

    Who Should Avoid Them?

    • Antihistamines can pose a danger for people with pre-existing medical conditions. Don't take antihistamines without permission from your doctor if you suffer from respiratory illnesses, such as asthma or emphysema; heart disease; high blood pressure; glaucoma; thyroid disease; or have trouble urinating.

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