Citalopram × Omeprazole

Omeprazole inhibits CYP2C19, which handles about 38% of citalopram metabolism. In therapeutic drug monitoring data, citalopram serum levels were about a third higher with omeprazole (+35%). The FDA caps citalopram at 20 mg/day with CYP2C19 inhibitors because QT prolongation is dose-dependent.

QT prolongation on the ECG. Clinically: dizziness, palpitations, syncope. Severe cases progress to torsade de pointes. The risk is higher with bradycardia, hypokalaemia, hypomagnesaemia, congenital long QT syndrome, recent myocardial infarction and decompensated heart failure.

The pair is not banned, but the citalopram dose is capped: with omeprazole or another CYP2C19 inhibitor, no more than 20 mg/day (US Food and Drug Administration (FDA) citalopram label). The Spanish citalopram SmPC asks for caution with omeprazole, and the omeprazole SmPC warns that it can raise levels of drugs cleared through CYP2C19. Potassium and magnesium are corrected before citalopram is started, and in patients at risk of electrolyte disturbances they are checked at baseline and periodically (citalopram labels). If signs of arrhythmia appear, citalopram is stopped and an electrocardiogram (ECG) is done (citalopram SmPC).