Histamine Page 5

Clinical Uses: H₁ Histamine Receptor Blockers
Motion Sickness: Scopolamine and certain first-generation H₁ blockers -- most effective drugs for motion sickness prevention Diphenhydramine and promethazine: greatest effectiveness Cyclizine (Marezine) and meclizine: also effective, less sedating Scopolamine and first-generation H₁ blockers are more effective in preventing motion sickness when given in combination with ephedrine or amphetamine return to main menu Nausea and Vomiting (Pregnancy) H₁ blockers are not recommended for use in management of nausea and vomiting associate with pregnancy because: Difficulty in assessment of possible birth defects associated with certain H₁ (benedictin) antagonists and known teratogenic effects of others (e.g., doxylamine) in animal models. return to main menu H₁ blockers: Toxicity Uncommon toxic effects following systemic demonstration: excessive excitation and convulsions in children orthostatic (postural) hypotension Allergic responses Drug allergy -- relatively common, following topical use of H₁ antagonists First-generation overdosage: similar to atropine overdosage Second-generation overdosage: may induce cardiac arrhythmias return to main menu Drug-Drug Interactions Second-generation H₁ blockers: Myocardial toxicity: Toxicity follows combination of terfenadine or astemizole combined with ketoconazole (Nizoral), itraconazole (Sporanox), or macrolide antibiotics (e.g.,erythromycin) because- Q-T (ECG) prolongation ventricular arrhythmias (potentially fatal) Terfenadine (Seldane)/astemizole (Hismanal) -- contraindicated for: patients taking ketoconazole (Nizoral), itraconazole (Sporanox), macrolide antibiotics, and patients with diminished liver function Fexofenadine (Allegra), a metabolite of terfenadine (Seldane), is safer. It is possible that terfenadine (Seldane) may be withdrawn (or has been withdrawn recently) from the market given availability of safer alternative medication. return to main menu