Both drugs prolong QT by blocking hERG potassium channels. Effect is additive with risk of torsades de pointes and ventricular fibrillation. FDA capped citalopram at 20 mg/day in patients over 60 in 2011 because of QT prolongation.
Symptoms
Palpitations, dizziness, syncope; ECG QTc above 500 ms. High sudden cardiac death risk with additional factors: hypokalaemia, hypomagnesaemia, bradycardia.
Management
Use alternative antiemetics when possible (metoclopramide, short-course domperidone). If ondansetron is needed, single IV dose no higher than 16 mg, ECG before and after, correct potassium and magnesium. Contraindicated in congenital long QT syndrome.
Check the full regimen, not just this pair
Opens the checker with these two drugs prefilled. Add the rest of the regimen and recompute additive risks.