Antihistamines associated with an increased risk of cardiac arrhythmias in patients receiving erythromycin or ketoconazole include

Terfenadine and astemizole are relatively new, nonsedating antihistamines that may prolong the QT interval of an electrocardiogram and cause cardiac arrhythmias. Erythromycin, ketoconazole, and itraconazole may inhibit the metabolism of terfenadine, resulting in higher blood levels and increased risk of arrhythmia. Loratadine, another nonsedating antihistamine, apparently lacks the QT prolongation effect and no drug interactions with either erythromycin or the antifungal agents listed have been established. Diphenhydramine, clemastine, and chlorpheniramine are also free of these drug interactions. However, many antihistamines have some potential to cause tachycardia and palpitations because of their antimuscarinic activity.