Major
citalopram × Quetiapine
SSRI (selective serotonin reuptake inhibitor)×Atypical antipsychotics
Mechanism
Quetiapine prolongs QT at high doses. Citalopram does so dose-dependently. Quetiapine depends on CYP3A4, citalopram on CYP2C19 and CYP3A4, so direct pharmacokinetic interaction is absent. The main risk is additive QT.
Symptoms
QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.
Management
The combination is acceptable at low doses of both (citalopram ≤20 mg, quetiapine ≤300 mg). ECG before start and at 2 weeks; maintain potassium and magnesium. Alternative antipsychotics with minimal QT effect: olanzapine or aripiprazole.