Many meds cause a prolonged QT; this is due to a mutation of a gene that codes for the rapid component of the K+ rectifying current. This leads to problems with repolarization.
Drugs causing prolonged QT with THERAPEUTIC doses include: antiarrhythmics (quinidine, procainamide, amiodarone, sotalol, and dofetilide)
Other agents that cause prolonged QT with ELEVATED serum concentrations include: antihistamines, some antibiotics and psychiatric meds (amitriptyline, cisapride, erythromycin, pimozide, thioridazine, SSRIs, trazodone, and moxifloxacin)
Use caution when combining medications from either, or both groups!
Other factors that are associated with prolonged QT include: bradycardia, female sex, genetics, and electrolyte abnormalities.
References
Kannakeril PJ, Roden DM. Drug-Induced long QT and torsade de pointes: recent advances. Current opinions in cardiology. Jan 2007;22(1):39-43.
Horn JR, Hansten PD. Drug Interactions and QT Interval Prolongation. Pharmacy Times Dec 2004. pp66.
