Published: June 2005

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Thioridazine Prescribing Reminder

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Prescriber Update 26(1): 9
June 2005

Medsafe Pharmacovigilance Team

Prescribers are reminded that thioridazine is a third-line antipsychotic agent and should only be prescribed in patients for whom the therapeutic benefits clearly outweigh the risk of arrhythmia from QT-prolongation.

Thioridazine (Aldazine^®, Melleril^®*) increases the risk of arrhythmia from QT-prolongation.  Consequently, the Medicines Adverse Reactions Committee (MARC) recommended changes to how thioridazine is prescribed in New Zealand.  The following advice was issued in June 2001 (see www.medsafe.govt.nz/profs/PUarticles/thioridazine.htm), which the MARC would like to reiterate.

Thioridazine is contraindicated in the following circumstances, all of which are risk factors for arrhythmia:

• use with medicines which inhibit the metabolism of thioridazine (cimetidine, most antidepressants, pindolol, propranolol)
• use with medicines which prolong the QT-interval (some antiarrhythmics, most antipsychotics, cisapride)
• use in patients with predisposing factors for arrhythmia or pre-existing QT-prolongation (QT_c ≥ 500ms).

Thioridazine should be initiated only by a specialist and only as third-line therapy.  In addition, the following precautions should be observed for new patients:

• assess for risk factors for arrhythmia
• check the QT_c-interval and serum potassium
• use the minimum effective dose
• observe a maximum of 200mg daily ordinarily
• use dosages above 200mg daily up to a maximum of 600mg daily only under specialist supervision and only with QT_c-interval monitoring following each increase.

For all patients currently taking thioridazine:

• assess for risk factors for arrhythmia
• check the QT_c-interval and serum potassium
• doses above 200mg daily require review by a specialist
• reduce the dose if QT_c-interval ≥ 500ms
• withdraw thioridazine if QT_c-interval is persistently ≥ 500ms, or in the presence of any contraindication.

Any children taking thioridazine should also have their therapy reviewed by a specialist.  If thioridazine discontinuation is necessary, gradually reduce the dose over a period of one month and concurrently introduce alternative medication.  Specialist supervision may be advisable for the withdrawal process.

* Novartis has advised that from 1 July 2005 supplies of Melleril will no longer be available.

Competing interests (authors): none declared.