antihistamines

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The antihistamines are a family of drugs that have many uses in the treatment of allergic disorders. If you have an allergy problem you'll likely use antihistamines once in a while and possibly for longer periods of time. Chances are you've used and will use many different antihistamines over time. As with any medication you take, it pays to know more about what you're putting into your body. This handout will discuss how antihistamines can help you, how they do it, how best to use them, and what adverse effects you may encounter in their use.

Pharmaceutical research has given us antihistamines with a variety of uses. The first antihistamine were developed by Halperin in 1943. These drugs have been around for as long as penicillin and fifteen years longer than Tylenol®. Antihistamines are now divided into two broad classes, H₁ and H₂ blockers, depending on which type of cells they effect. H₂-blockers, like Tagomet® and Zantac®, decrease acid production in the stomach. They're useful for the treatment of peptic ulcer, hiatal hernia, and other hyperacidity conditions. The H₁-blockers are the antihistamines that we usually take for problems like allergic rhinitis or allergic conjunctivitis (hay fever), hives, and eczema (atopic dermatitis).

Antihistamines attach themselves to histamine receptors on cell surfaces. This competitive inhibition prevents histamine molecules from attaching to the receptors consequently inhibiting activation of the cell and interrupting the sequence of the allergic reaction. Histamine is a messenger substance released by the mast cell. Mast cells are the lynch pin cell in the type of allergic reaction called immediate hypersensitivity. This is the type of allergic reaction that characterizes hay fever, asthma, hives, and anaphylaxis.

Mast cells release histamine and other substances that control local blood flow and attract other cell types of the immune system to the area. Mast cells can be activated by a variety of mechanisms. IgE antibodies are a special class of antibody that attaches itself to receptors on the mast cell. If a foreign substance (an antigen) attaches itself to the bound IgE antibody, the mast cell becomes activated leading to histamine release and initiation of an allergic reaction. Everyone makes some IgE antibodies but allergic persons make much larger amounts of them. They make them to many different but otherwise innocuous substances in our environment. Antihistamines do not prevent the development of IgE antibodies or even the activation of mast cells. They block the action of histamine, a primary effector substance released by the mast cell. Since mast cells release many other active chemicals, antihistamines cannot completely block an allergic reaction; they simply diminish it.

Many effective antihistamines are available over-the-counter. Their advantages are availability and low cost. These drugs include chlorpheniramine maleate (present in Triaminic®, Pediacare®, Coricidin®,, tripolidine maleate (Actifed®), brompheniramine maleate (Dimetapp®, Dimetane®), diphenhydramine (Benadryl®), and clemastine (Tavist®). Most of these agents are rapidly absorbed and quickly eliminated from the body making them advantageous for treatment of sudden, brief allergic reactions. Tavist is longer acting. Many of the others are sold in formulations designed to prolong their action. This is useful for treating more chronic conditions like seasonal allergy symptoms. These antihistamine also have antiemetic properties making them useful for treating motion sickness and nausea.

Some antihistamine are better suited for a special purpose than others. Atarax® (hydroxysine) is particularly effective at suppressing itching. It is the drug of choice for treatment of hives, angioedema, and eczema. Meclizine (Bonine®) is superior to other antihistamines for dizziness. It is a popular medication for motion sickness.

The major disadvantage of these antihistamines is their tendency to cause central nervous system depression, drowsiness, and sedation. Sedation may be the more serious side effect for drivers, pilots, and persons who operate machinery as it slows one's reaction time. Poor reaction time may result in accidents and injuries. The soporific properties of antihistamine should not be minimized. For example, Sominex®, a product for insomnia, is diphenhydramine, the same drug as Benadryl. Fortunately, only 20-25% of users of these antihistamine will experience drowsiness or altered reaction time. For those that do, continued use for several days often results in adaptation i.e. you no longer feel quite as sleepy as when you first took the medication. Nevertheless, it's a good idea to avoid other drugs that may make you drowsy or effect your judgment or timing when you're taking a classical antihistamine. Such drugs include alcohol, tranquilizers, and muscle relaxants.

Terfenadine (Seldane®), astemizole (Hismanal®), fexofenadine (Allegra®), and loratidine (Claritin®) are representatives of a new group of long-acting, non-sedating antihistamines. These drugs cause somnolence and sedation no more frequently than does placebo (about one in fifty patients complain of sedation when using one of these drugs). Seldane and Allegra may be used twice a day while the other two need only be taken once a day to be effective.

Because both Seldane and Hismanal are associated with a rare but potentially fatal disorder of the heart called "Toursades de Pointes" they are no longer available for use. Claritin and Allegra have not been associated with heart arrhythmia or other heart problems.

Another new, effective long-acting antihistamine is cetirizine (Zyrtec®). Zyrtec is intermediate in the frequency with which it causes drowsiness. Where the non-sedating antihistamines are associated with sedation in about 2% of users, Zyrtec causes drowsiness 14% of the time it is taken.

Astelin® is a unique antihistamine nasal spray. Because this medication is sprayed directly into the nose, it has a rapid onset of action. Sedation has been reported in up to 19% of users. Some users also complain if its unpleasant taste.

is the three to five pound weight gain sometimes seen with chronic Seldane or Hismanal use.

Remember that any drug can induce unexpected adverse effects in susceptible individuals. Unusual side effects of antihistamines include stomach upset, difficulty urinating (especially in older patients), and hallucinations. If you suspect that your antihistamine is causing any sort of side effect, stop the drug for several days and see if the problem resolves. You may want to test the relationship between drug and side effect by taking the antihistamine for a second time to see if the adverse effect returns.

Antihistamines work more effectively if taken before symptoms begin. That's why for hives, eczema, and allergic rhinitis regular use is recommended, at least for several weeks at a time. If symptoms are intermittent, for example allergic rhinitis only when mowing the lawn or when visiting a home with a cat, take the medication an hour or two beforhe exposure for optimal effectiveness.

After an antihistamine is used regularly for months, tolerance to its effects may develop. This diminished effectiveness is usually only temporary and can be reversed by avoiding the medication for a few weeks. Another antihistamine may be substituted during this time and will work well since tolerance is drug-specific.

Some antihistamines are marketed as combination products with decongestants as a weakness of antihistamine therapy is poor relief of nasal congestion. Decongestants can sometimes induce jitteriness and wakefulness.

Antihistamines interfere with allergy skin tests and should be avoided before testing. The period varies from 24 hours for most classical antihistamines to four to six weeks for Hismanal. Other long-acting antihistamines should be avoided for seven days prior to skin testing. Check with us to determine how long you must be off your antihistamine before testing can be performed successfully.

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The information above represents the writer's opinions and is not meant as a substitute for evaluation by a physician. The reader is advised to seek sound medical evaluation and guidance before undertaking treatment for any medical condition.