Antihistamines are primarily used in the treatment of urticaria and for the amelioration of pruritus resulting from various causes. Two major categories of antihistamines are available
-H1 blockers
-H2 blockers.
The approximately 30 H1 antagonists are subdivided into several chemical groups, but there is relatively little clinical difference among them. Familiarity with three or four different H1 antihistamines should be sufficient for the treatment of most problems. Diphenhydramine (Benadryl) in a dosage of 100 to 200 mg/ day is the most widely used antihistamine. It is quite potent, but its use is accompanied by considerable sedation. Chlorpheniramine (Chlor-Trimeton, Teldrin) in a dosage of 12 to 16 mg/day is equally potent. Both of these agents are inexpensive and are available over the counter. Cyproheptadine (Periactin) in a dosage of 12 to 16 mg/day is said to have an antiserotonin effect as well as an antihistamine effect, this may give it a somewhat broader anti-inflammatory effect. Azatadine (Optimine) and clemastine (Tavist) are two other, less sedating, frequently used antihistamines. Hydroxyzine (Atarax, Vistaril) in a dosage of 100 to 200 mg/day protects better against histamine-induced wheal formation than any other antihistamine. Hydroxyzine also has a significant tranquilizing effect. Overall, it is the antihistamine most widely preferred by dermatologists in the treatment of urticaria and pruritus generally.
The dosages given above for these antihistamines can be viewed as approximations. The key to their successful use is to advance the dose gradually until clinical improvement occurs or until sedation or anticholinergic side effects (dry mouth, blurred vision, and impaired micturition) become troublesome. Some sedation occurs with administration of all of the above antihistamines, but this can be minimized by having patients take most of their daily dose at suppertime and bedtime. Generally, sedation becomes less noticeable after the first week of therapy. Patients should be warned about lengthened reaction times while driving or operating machinery. Alcohol increases the severity of these problems and should be avoided when the above agents are used.
Terfenadine (Seldane) administered in a dose of 60 mg twice a day and astemizole (Hismanal) given as 10 mg once a day are essentially nonsedating and are therefore associated with high patient compliance. These agents are very useful in the treatment of urticaria but unfortunately, probably because of their minimal central nervous system (CNS) effects, have little or no antipruritic effect in other settings. Both drugs are fairly expensive and, when given in larger than normal doses, may be associated with cardiac conduction abnormalities. They should not be used concomitantly with erythromycin or ketoconazole, since these latter agents elevate senlm levels of both antihistamines. Two new, nonsedating H1 antihistamines (cetirizine and loratadine) are expected to be available in the United States soon, but it seems unlikely that they will prove markedly superior to the currently available drugs.
Several H2 antagonists are available, the two most widely used are cimetidine (Tagamet) and ranitidine (Zantac). Although marketed for the treatment of ulcerative gastrointestinal disease, they are used by dermatologists for the treatment of H1-resistant chronic urticaria. When used alone, H2 blockers have no effect on vascular dilation or permeability, but when used for urticaria, they do seem to act synergistically with HI blockers. Cimetidine also has interesting antiandrogenic and, possibly, enhancing effects on cell-mediated immune response. Unfortunately, because of its effect on the cytochrome P-450 system, it is also associated with numerous drug interactions.
Tags:anticholinergic side effects, antihistamines, azatadine, cyproheptadine, Diagnostic and Therapeuti Techniques, diphenhydramine benadryl, hydroxyzine atarax, pruritus urticaria
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