Potentially fatal interaction between azithromycin and disopyramide

A patient on disopyramide developed disopyramide toxicity when treated concurrently with azithromycin. Evidence of toxicity included an elevated serum disopyramide level and ventricular tachycardia requiring cardioversion. The azalide antibiotic presumably inhibited dealkylation of disopyramide to its major metabolite, mono-N-dealkyldisopyramide. Physicians should avoid using azithromycin in patients on disopyramide. If this drug combination is unavoidable, disopyramide levels must be closely monitored.