 Other medications may increase the risk for a low level of potassium in the blood, thereby indirectly increasing the risk for QT prolongation.For example, dexamethasone, hydrochlorothiazide, and theophylline can lower the level of potassium in the blood.Lofexidine can depress the central nervous system (CNS), which, in combination with other CNS depressants, may reduce a person's ability to perform tasks that require skills and attention.