Antihistamines and Decongestants

Histamine is a substance present in various tissues of die body, such as die heart, lun, gastric mucosa, and skin (Pig. 36-1). The Mgliest concentration of histamine is found in die basopMl (a tyiie of wlnte blood cell) and mast cells diat are found near capillaries. Histamine is produced in response to injury. It acts on areas such as die vascular system and smoodi muscle, producing dilatation of arterioles and an increased iienneability of capillaries and venules. Dilatation of die arterioles results in localized redness. An increase in die iienneability of 

Antihistamines are dru used to counteract the effects of histamine on body organs and structures. Elxampl of antihistamines include diphenhydramine (Benadryl), loratadine (Claritin), fexofenadine (All ra), and cetirizine (Zyrtec). A new antihistamine, d lorata-dine (Clarinex), is die active metabolite of loratadine and is intended to eventually replace loratadine (Claritin). Topical corticosteroid nasal sprays such as fluticasone propionate (Flonase) or triamcinolone ace-tonide (Nasacort AQ) are also used for nasal allergy symptoms. See (3iapter 56 for more information on die topical corticosteroids. 

Antihistamines block most, but not all, of die effects of histanhiia They do this by comiietiiig for histamine at histamine receptor sites, diereby preventing histamine 

Relief of allergic reactions to drugs, blood, or plasma 

Each antihistamine may be used for one or more of these reasons. The more specific uses of the various antihistamine preparations are given in tlie Summary Drug Table Antihistamines. 

Drowsiness and sedation are common adverse reactions seen with the use of many of the antihistamines. Some antihistamines appear to cause more drowsiness and sedation than others. These dm may also have varying degrees of anticholinergic (cholinergic blocking) effects, which may result in dryness of the mouth, nose, and throat and a thickening of bronchial secretions. Several newer preparations (eg, loratadine) cause little, if any, drowsiness and fewer anticholinergic effects than some of the other antihistamines. Hiotosensitivity may occur with the use of the antihistamines. 

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The principal OTC pharmaceutical products include cold remedies, vitamins and mineral preparations, antacids, analgesics, topical antibiotics, antiftingals and antiseptics, and laxatives. Others include suntan products, ophthalmic solutions, hemorrhoidal products, sleep aids, and dermatological products for treatment of acne, dandmff, insect parasites, bums, dry skin, warts, and foot care products (11). More recent prescription-to-OTC switches have included hydrocortisone, antihistamine and decongestant products, antiftingal agents, and, as of 1995, several histamine H2-receptor antagonists. 

Unit V has three chapters concerning drugs that affect the respiratory system. The first chapter in this unit discusses antihistamines and decongestants, the second chapter in the unit covers bronchodilators and antiasthma drugs, and the last chapter of the unit deals with antitussives, mucolytics, and expectorants. 

Pharmacotherapy has an important role in managing AR symptoms (Table 59-2). Intranasal corticosteroids, systemic and topical antihistamines and decongestants, mast cell stabilizers, and immunotherapy all are beneficial in treating symptoms of AR.9 Antihistamines and intranasal corticosteroids are considered first-line therapy for AR, whereas decongestants, mast cell stabilizers, leukotriene modifiers, and systemic corticosteroids are secondary treatment options10-12 (Fig. 59-2). Whenever exposure to allergens can be predicted (e.g., SAR or visiting homes with a pet), medications should be used pro-phylactically to maximize effectiveness.11... 

Oral Dosages of Commonly Used Antihistamines and Decongestants... 

OTC with pamabrom + pyrilamine (Midol PMS, Pamprin) with antihistamine and decongestant (Actifed Plus, Drixoral Cold Flu, Benadryl Sinus, Sine-Off, Sina-rest) with decongestant, antihistamine, dextromethorphan (Nyquil)... 

Figure 9.16. Separation of a mixture of antihistamines and decongestants by ion pair chromatography. Courtesy of Millipore Corp., Waters Chromatography Division.

FIGURE 5-20. Structures of various antihistamines and decongestants. These compounds are referenced in Figures 5-21 to 5-24. 

FIGURE 5-23. Chromatogram showing the separation of antihistamines and decongestants (refer to Figure 5-20) using hexane sulfonate. Mobile phase methanol/water (50 50) with 5 mM hexane sulfonate and 1% acetic acid. UV detector 254 nm. Flow rate 2 mL/min. (Reprinted from reference 2 with permission.)... 

Practitioners should be aware of over-the-counter (OTC) medications and folk or home remedies that patients may be using. Many patients may not consider OTC agents, especially antihistamines and decongestants for hay fever and colds, as drugs. These can affect the autonomic nervous system. OTC preparations can potentially interact with ocular drugs, such as homatropine and phenylephrine, that also influence autonomic functions. 

Weisman RS. Antihistamines and decongestants. In Goldfirank LR, Flomenbaum NE, Lewin NA, Howland MA, Hoffman RS, Nelson LS, eds. Goldfrank s toxicologic emergencies, 7 ed. New York McGraw-Hill, 2002 535-43. 

Less serious allergic rhinitis and hay fever are treated mainly with antihistamines and decongestants. 

Describe the uses, general drug adion, general adverse readions, contraindications, precautions, and interactions of the antihistamines and decongestants. 

Many antihistamines and decongestants are soid as their hydrochioride saits. 

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