Drug Interaction Report

MONITOR: Tizanidine has alpha-adrenergic agonist activities and may potentiate the hypotensive effects of other medications, including alfuzosin. In addition, both alfuzosin and tizanidine may cause prolongation of the QTc interval. Theoretically, coadministration of agents that can prolong the QT interval may result in elevated risk of ventricular arrhythmias, including ventricular tachycardia and torsade de pointes, because of additive arrhythmogenic potential related to their effects on the QT interval. Tizanidine has been associated with QT interval prolongation during toxicity studies with dogs. During a clinical study involving 45 healthy male subjects, average prolongation of QTc interval at peak plasma concentrations was less with single doses of alfuzosin 10 mg than with alfuzosin 40 mg, and both were less than with moxifloxacin 400 mg. Heart rate increased by an average of 5.2 bpm with alfuzosin 10 mg, 5.8 bpm with alfuzosin 40 mg, and 2.8 bpm with moxifloxacin 400 mg. Neither drug has been associated with torsade de pointes in humans.

MANAGEMENT: Caution and clinical monitoring are recommended during concurrent therapy. Patients should be advised to notify their physician if they experience orthostatic symptoms (e.g., lightheadedness, dizziness, syncope) and to seek medical help if they experience palpitations, or irregular heartbeats.

ADJUST DOSING INTERVAL: Administration of alfuzosin with food enhances oral bioavailability. According to the manufacturer, extent of absorption is 50% higher when administered under fed conditions compared to fasting conditions.

MANAGEMENT: To ensure maximal oral absorption, alfuzosin should be administered with or immediately after a meal.