Polymorphic ventricular tachycardia can occur when terfenadine(antihistaminic):

Torsades de pointes (polymorphic ventricular tachycardia) occurs when terfenadine is taken in higher doses or when hepatic metabolism is impaired by disease or by drugs which inhibit the cytochrome P450 responsible for metabolism of terfenadine. Drugs include erythromycin, clarithromycin, ketoconazole and itraconazole. Azithromycin and fluconazole that are excreted unchanged in the urine have not been associated with impaired metabolism of terfenadine. Terfenadine blocks the delayed rectifier potassium channels and prolongs cardiac repolarization and the QT interval.