Azithromycin × Ketoconazole

Additive QT prolongation.

QT prolongation on the ECG, dizziness, fainting, a fast or irregular heartbeat. Rarely, polymorphic ventricular tachycardia (torsades de pointes) with a risk of cardiac arrest. Risk is higher with low potassium or magnesium, clinically relevant bradycardia, arrhythmias, severe heart failure, in women and in older patients (azithromycin SmPC).

Both drugs prolong the QT interval (azithromycin SmPC; US ketoconazole label). Azithromycin is used with caution in patients taking other QT-prolonging drugs and in those with QT prolongation, low potassium or magnesium, clinically relevant bradycardia, arrhythmias or severe heart failure (azithromycin SmPC). In the EU, oral ketoconazole is authorised only for Cushing's syndrome; its SmPC asks for an ECG before the start and within the first week of treatment, and recommends ECG monitoring when a QT-prolonging drug is added (ketoconazole SmPC, EMA). Congenital or acquired QT prolongation is a contraindication to ketoconazole (ketoconazole SmPC, EMA). Tell the patient to report fainting, dizziness or a fast, irregular heartbeat at once (US ketoconazole label).