Citalopram × Pimozide

Both drugs prolong the QT interval, and both labels assume the effects add up. Both labels cite the same study: in subjects on citalopram 40 mg once daily for 11 days, a single 2 mg dose of pimozide increased QTc by a mean of about 10 ms compared with pimozide alone. The labels differ on pharmacokinetics. The pimozide label states that citalopram did not change pimozide AUC or Cmax; the Spanish citalopram label reports a rise that was not consistent through the study. The pimozide label calls the mechanism of this pharmacodynamic interaction unknown.

QT prolongation itself causes no symptoms and shows only on the ECG. The danger is ventricular arrhythmia, above all torsade de pointes. Both labels report such cases, and the pimozide label also reports ventricular tachycardia, ventricular fibrillation and sudden death, including at therapeutic doses. Arrhythmia presents as dizziness, palpitations or syncope (citalopram label, FDA). The citalopram label reports torsade de pointes mainly in women and in patients with hypokalaemia, a pre-existing long QT interval or heart disease.

The combination is contraindicated in all four labels. The Spanish pimozide label names citalopram in its contraindications (pimozide label, AEMPS). The Spanish citalopram label bans concomitant use in a dedicated paragraph because the interaction appeared at a low pimozide dose (citalopram label). The US labels of both drugs repeat the ban (citalopram and pimozide labels, FDA). Do not give citalopram to a patient on pimozide who needs an antidepressant; if a patient already takes both, stop one of the drugs. The ban also covers other serotonin reuptake inhibitors, named as sertraline, paroxetine and escitalopram (pimozide label), and any drug that prolongs the QT interval (pimozide and citalopram labels). Choose the replacement outside those lists. For any pimozide treatment, take an ECG before the start and repeat it during treatment, correct potassium and magnesium, and stop pimozide if QT or QTc exceeds 500 ms (pimozide label).