amiodarone and azithromycin

Both amiodarone and azithromycin can prolong the QT interval. Amiodarone blocks multiple cardiac ion channels, including the rapid delayed rectifier potassium current (IKr), while azithromycin also inhibits IKr. The pharmacodynamic effect is additive, leading to a significant prolongation of the QT interval and an increased risk of torsades de pointes, a potentially fatal ventricular arrhythmia.

Co-administration increases the risk of QT interval prolongation, torsades de pointes, and sudden cardiac death. The risk is higher in patients with pre-existing QT prolongation, bradycardia, hypokalaemia, hypomagnesaemia, or other risk factors for arrhythmia. The effect may be delayed due to amiodarone's long half-life.

Avoid concurrent use if possible. If co-administration is necessary, use the lowest effective doses and closely monitor the QT interval with electrocardiography before and during therapy. Assess and correct electrolyte imbalances. Consider alternative antibiotics that do not prolong the QT interval. Seek specialist advice if the patient has additional risk factors.

Well-established interaction based on pharmacological principles, case reports, and observational studies. Guidelines on QT-prolonging drugs frequently list this combination as a significant risk.