In common use, the term "antihistamine" refers only to H In type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface Ig E antibodies on mast cells and basophils.
Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil.
Since older antihistamines can impair mental and motor functioning, they can impair a person’s ability to operate motor vehicles or heavy machinery.
In fact, in many states, a person can be charged with driving-under-the-influence (DUI) if they operate a motor vehicle while taking medications such as Benadryl.
Learn everything you ever wanted to know about Benadryl.
Two newer, less-sedating antihistamines, Claritin and Zyrtec, are also available OTC without a prescription.
Oral antihistamines can also help treat the symptoms of eye allergies.
Antihistamines are also available as nasal sprays for the treatment of allergic rhinitis (nasal allergies) and as eye drops for the treatment of allergic conjunctivitis (eye allergies).
Other Uses for Antihistamines Antihistamines are frequently used for the treatment of other allergic conditions, although they may not be as effective as they are for hay fever and hives.
These conditions may include: Antihistamines Available Over-the-Counter (OTC)Probably the most well-known oral antihistamine is Benadryl, which is an older, sedating antihistamine.
Antihistamines are commonly used for the treatment of various allergic diseases, and can be taken in oral, nasal spray, eye drop, and injected forms.
Antihistamines for Hay Fever (Allergic Rhinitis)Newer oral antihistamines, such as Xyzal (levocetirizine), Zyrtec (cetirizine), Allegra (fexofenadine), Clarinex (desloratadine) and Clarinex (loratadine), are very effective therapies for the treatment of hay fever.
As of the writing of this article in December 2010, Allegra is also only available by prescription but is due to be available OTC during the Spring of 2011.