Which antibiotic classes are commonly associated with QT interval prolongation?

• A. Tetracyclines and beta-lactams.
• B. Macrolides and fluoroquinolones.
• C. Glycopeptides and oxazolidinones.
• D. Aminoglycosides and polymyxins.

Answer: (B)

QT interval prolongation from antibiotics happens when a drug interferes with the heart’s repolarization phase, often by blocking the hERG potassium channels that underlie the rapid component of the delayed rectifier current. This delay can extend the QT interval on the ECG and, in susceptible patients, lead to torsades de pointes, a dangerous form of ventricular tachycardia.

The antibiotic classes most commonly linked to this effect are macrolides and fluoroquinolones. Macrolides, historically including erythromycin and some others, have well-documented cases of QT prolongation, and fluoroquinolones like levofloxacin and especially moxifloxacin also carry a notable risk. The combination of these two classes accounts for the strongest and most common association with QT prolongation.

Other antibiotic groups—tetracyclines, beta-lactams, glycopeptides, oxazolidinones, aminoglycosides, and polymyxins—do not have QT prolongation as a typical or prominent drug-class effect. Their adverse effect profiles center more on other toxicities (for example, ototoxicity or nephrotoxicity with aminoglycosides; nephrotoxicity with polymyxins; different metabolic or hematologic effects with others) rather than meaningful QT prolongation.

In practice, when using macrolides or fluoroquinolones in patients with risk factors for QT prolongation (electrolyte abnormalities, congenital long QT, heart disease, bradycardia, or concurrent QT-prolonging drugs), monitoring the ECG and electrolytes and considering alternative antibiotics if feasible helps mitigate the risk.