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Macrolide antibiotics can affect the QT interval by prolonging ventricular repolarization. They do this mainly by blocking the hERG potassium channels, which reduces the IKr current and delays the return of the ventricles to their resting electrical state. The result on the ECG is a longer QT interval, which raises the risk of torsades de pointes, a dangerous form of ventricular tachycardia. This risk is established with drugs like erythromycin and can occur with other macrolides to varying degrees, especially when combined with other QT-prolonging meds or in patients with electrolyte disturbances, congenital long QT, or bradycardia. So yes, macrolide antibiotics can affect the QT interval.