Major
citalopram × Clarithromycin
SSRI (selective serotonin reuptake inhibitor)×Macrolide antibiotic
Mechanism
Clarithromycin prolongs QT and is a potent CYP3A4 inhibitor. Citalopram is metabolised by CYP3A4 only minorly, so pharmacokinetic effect is small, but QT prolongation adds up.
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
Avoid the combination. Alternative antibiotics: azithromycin (weaker QT effect) or doxycycline/a cephalosporin. If clarithromycin is needed: do not increase citalopram; ECG before start and every 3 days during therapy.