Terfenadine is also contraindicated in patients having known QT prolongation (corrected QT, QTc > 440 ms), e. g. congenital long QT Syndrome, or conditions which may lead to QT prolongation, such as − clinically significant bradycardia − history of symptomatic arrhythmias − any other clinically significant cardiac disease − concomitant treatment with Class I or III anti-arrhythmics − concomitant treatment with other medicinal products known to prolong the QT interval These are also listed under 4.5 (Interactions). − electrolyte imbalance, particularly hypokalemia or hypomagnesemia, and medical conditions or concomitant treatment with drugs with the potential of inducing such imbalance.