Atomoxetine × Levofloxacin

Both can lengthen the QT interval and the effect adds up. The atomoxetine label asks for caution in patients with congenital or acquired long QT and with a family history. The levofloxacin label warns that fluoroquinolones should be used with caution where QT prolongation risk factors exist, and explicitly mentions concomitant use of QT-prolonging medicines.

Palpitations, dizziness, a feeling of impending faint or syncope. The serious long-QT arrhythmia, torsade de pointes, can present directly with loss of consciousness. Hypokalaemia and hypomagnesaemia, common with vomiting or diarrhoea, increase the risk.

For a common infection, an antibiotic without QT effect avoids the problem; levofloxacin is reserved for when there is no alternative. If the pair is necessary, correct potassium and magnesium before starting and consider an ECG in patients with heart disease, a family history of sudden death or other QT-prolonging drugs.