amiodarone and methadone

Both amiodarone and methadone prolong the QT interval by blocking cardiac repolarising potassium currents, particularly the rapidly activating delayed rectifier current (Ikr) mediated by the hERG channel. Co-administration results in additive QT prolongation and an increased risk of torsades de pointes. Amiodarone may also inhibit the metabolism of methadone via CYP3A4 and CYP2D6, further increasing methadone concentrations and its QT-prolonging effect.

Concurrent use significantly increases the risk of life-threatening ventricular arrhythmias, including torsades de pointes and sudden cardiac death. The risk is greater in patients with pre-existing QT prolongation, hypokalaemia, hypomagnesaemia, bradycardia, or those taking other QT-prolonging drugs. Symptoms may include palpitations, syncope, or seizures.

Avoid co-prescription where possible. If the combination is necessary, obtain a baseline ECG and electrolyte measurements (potassium, magnesium), and repeat ECG regularly during therapy, especially after dose changes. Correct any electrolyte abnormalities. Consider alternative therapies that do not prolong the QT interval. Monitor for signs of arrhythmia and advise the patient to report palpitations or syncope.

This interaction is well established and documented in multiple clinical guidelines, including those from the American Heart Association and the European Society of Cardiology, which list both drugs as having a known risk of torsades de pointes. Case reports and observational studies support the additive QT effect.