Drugs known to prolong QT interval or cause electrolyte imbalance. Mild to moderate increased conc w/ substrates or inhibitors of CYP3A4 or CYP2D6 isozymes eg, itraconazole, nefazodone, buspirone, venlafaxine, alprazolam, fluvoxamine, quinidine, fluoxetine, sertraline, chlorpromazine & promethazine. Increased QTc in combination w/ ketoconazole & paroxetine. Reduced plasma levels w/ prolonged treatment w/ enzyme-inducing drugs eg, carbamazepine, phenobarb, rifampicin. Increased CNS depression w/ other CNS-depressant drugs including alcohol, hypnotics, sedatives or analgesics. Enhanced CNS effect w/ methyldopa. May antagonize the action of adrenaline & other sympathomimetic agents & reverse the BP-lowering effects of adrenergic-blocking agents eg, guanethidine. May impair the antiparkinsonian effects of levodopa. Increased plasma levels of TCAs. Concomitant use w/ lithium. May antagonize effect of phenindione.