Among conventional antipsychotics, which agent carries the greatest risk of dose-related QT prolongation and torsades de pointes?
- A Haloperidol
- B Trifluoperazine
- C Thioridazine ✓
- D Fluphenazine
Correct answer: C. Thioridazine
Explanation
Thioridazine blocks hERG potassium channels most potently among older antipsychotics, causing dose-dependent QT prolongation and torsades de pointes, which led to major restrictions on its use. Haloperidol also prolongs QT, particularly intravenously, but to a lesser degree than thioridazine. Trifluoperazine and fluphenazine have comparatively low QT liability. In exam convention, thioridazine is quoted as the antipsychotic with the highest risk of fatal ventricular arrhythmias.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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