Two cases with QT prolongation associated with the administration of standard drug doses are reported. Drug effects are determined by pharmacological and biological parameters. Drugs may be metabolized or transported by two systems: the cytochrome enzymes or the less well-known but nevertheless relevant P-glycoprotein system. P-glycoprotein is a pump that limits intracellular reabsorption and favors the elimination of exogenous substances. These two systems interact with each other to influence drug effects. A better knowledge of their mechanisms of action and their genetic determinants should help provide more individualized pharmacologic treatment to patients, in particular older patients.