Major
Haloperidol × Mirtazapine
First-generation antipsychotics. Butyrophenone derivatives×Other antidepressants (NaSSA)
Mechanism
Mirtazapine prolongs QT dose-dependently, additively with haloperidol (especially parenteral). Plus combined sedation.
Symptoms
QT prolongation on ECG. Dizziness, syncope, palpitations. Severe cases: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia and hypomagnesaemia.
Management
Avoid the combination with mirtazapine above 30 mg and parenteral haloperidol. At low doses and normal potassium, the combination is acceptable with ECG monitoring at 2 weeks. Alternative antipsychotics with minimal QT effect: olanzapine or aripiprazole.