32 - Antihistamines

Abstract

Antihistamines bind to either H1 or H2 histamine receptors, thereby blocking histamine’s effects in the body. These medications act as inverse agonists, rather than receptor antagonists, by downregulating the activated state of histamine receptors. Histamine release from cutaneous mast cells causes itching and a hive-like reaction that is blocked by first- and second-generation H1 antihistamines, but not H2 antihistamines. Second-generation H1 antihistamines offer advantages over first-generation agents by causing less sedation, having a longer half-life and having little, if any, anticholinergic effects. Thus, second-generation H1 antihistamines are the preferred treatment for chronic idiopathic urticaria and allergic rhinitis. Although H1 antihistamines are frequently used in the treatment of atopic dermatitis, there is little evidence to support their therapeutic value. Some H1 antihistamines can be used safely in pregnancy. Although H2 antihistamines are rarely used in dermatology, knowledge of their potential side effects is important because they inhibit the hepatic cytochrome P-450 (CYP) system, thus increasing their risk for drug interactions. Doxepin, a tricyclic antidepressant, binds to both H1 and H2 receptors and is at times substituted for H1 antihistamines in allergic reactions. However, this medication should be used with caution in older patients because it can cause sedation.