Major
Amiodarone × Ciprofloxacin
Class III antiarrhythmics (Vaughan Williams)×Fluoroquinolone antibacterials
Mechanism
Additive QT prolongation. Amiodarone strongly blocks IKr potassium channels, ciprofloxacin weakly but dose-dependently. Ciprofloxacin also inhibits CYP1A2 (a minor amiodarone clearance route).
Symptoms
QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.
Management
Avoid where possible. Alternative antibiotic: a cephalosporin (no QT effect). If ciprofloxacin is needed, ECG before start and at day 3, with potassium and magnesium maintained.
Sources
- Lexicomp: Lexicomp Drug Interactions (2024)– Wolters Kluwer Clinical Drug Information, Inc. Lexi-Interact Online, 2024
- Pharmaceutical Press: Stockley's Drug Interactions, 12th edition (2024)– Preston CL (ed.). Stockley's Drug Interactions. 12th ed. London: Pharmaceutical Press; 2024
- FDA: FDA Drug Safety Communication: FDA updates warnings for oral and injectable fluoroquinolone antibiotics due to disabling side effects (2016)– FDA Drug Safety Communication, July 26, 2016