Allergic reactions involve an interaction between IgE, allergens, and mast cells or basophils, which trigger a release of chemical mediators such as histamine, leukotrienes, and prostaglandins.  After being released from mast cells, histamines seek out “receptor” sites located in the nasal lining tissue.  Like a key inserted into a lock, the histamine (key) links up with a receptor site (lock) and triggers allergic symptoms such as sneezing and itching.

First generation antihistamines (over-the-counter) have been used to treat allergic symptoms since 1942.  Antihistamines counter the effects of histamine because they attach themselves to the receptors before histamine.  Receptors accept only one chemical at a time, so if antihistamines block histamine, allergic symptoms will not be triggered.

The most common antihistamine medications are first-generation or nonprescription products available over-the-counter.  Most of these products, while bearing different brand names, contain similar active ingredients such as Brompheniramine maleate (Dimetapp), Chlorpheniramine maleate (Chlor-Trimeton), Clemastine fumrate (Tavist-1), and Diphenhydramine hydrochloride (Benadryl).

Although first generation OTC antihistamines can relieve symptoms of allergic rhinitis, they also can make one drowsy by crossing the blood-brain-barrier and affecting histamine receptors in the central nervous system.  Other side effects of first generation OTC antihistamines can include dry mouth, gastrointestinal distress, and urinary retention.

Due to noteworthy advances in research, several newer, second generation antihistamines have been developed such as Xyzal, Zyrtec, Allegra, Claritin, Clarinex, and Alavert.  These drugs not cross the blood-brain-barrier, and are therefore are less sedating, if sedating at all.  Unfortunately, these second generation antihistamines are generally more expensive.

Typically, antihistamines work best when taken on a regular basis before exposure to the offending allergen.  If an allergy to animal dander is known, take an antihistamine before visiting a friend or family member that has a dog or cat.  Antihistamines normally do not decrease nasal congestion, so they are frequently combined with a decongestant (D) such as Allegra-D, Zyrtec-D, or Claritin-D.  The most commonly used decongestants are pseudoephedrine and phenylephrine.  Common side effects of oral decongestants are sleeplessness, nervousness, agitation, dryness of the mouth, difficulty urinating, high blood pressure, and heart palpitations.