Case reports describing clarithromycin and its probable effect on digoxin metabolism have been sporadically described in the literature [1, 2]. These findings have important implications for patient care because both digoxin and clarithromycin are commonly prescribed medications. We report a case of digoxin toxicity during clarithromycin therapy.

A 70-year-old woman had been receiving digoxin, 0.25 mg/d, for the past 4 years for idiopathic cardiomyopathy. Other medications included hydrochlorothiazide, 25 mg/d; isosorbide mononitrate, 30 mg/d; and captopril, 50 mg every 8 hours. The patient began receiving clarithromycin, 500 mg twice daily, for bronchitis. After 4 days of this treatment, the patient was hospitalized because of nausea, vomiting, weakness, and the presence of brown spots in the patient's field of vision for 4 days. Physical examination was unremarkable except for mild dehydration. An electrocardiogram showed sinus rhythm, borderline first-degree atrioventricular block, and left bundle-branch block. The serum digoxin level was 4.8 ng/mL (normal, 0.8 to 2.4 ng/mL) on admission and increased to 5.4 ng/mL 2 hours after admission. On admission the blood urea nitrogen level was 29 mg/mL, creatinine level was 1.4 mg/dL, and potassium level was 4.9 mEq/L.

Digoxin and clarithromycin therapies were discontinued, and the patient was admitted to a telemetry bed. In 2 days, her symptoms improved, and the digoxin level decreased to 1.7 ng/mL. The medical record showed that 6 months before admission, the digoxin, blood urea nitrogen, creatinine, and thyroid-stimulating hormone levels were normal. At hospital discharge, the patient's digoxin level was 1.7 ng/mL and the electrocardiogram had not changed. Therapy with digoxin, 0.125 mg every other day, was restarted, and the digoxin level was 0.8 ng/mL 1 week later.

Erythromycin prolongs the clearance of digoxin and allows more of an active drug to be absorbed by altering the colonic flora that metabolizes digoxin to various inactive compounds [3]. We believe that clarithromycin has a similar effect on digoxin. Serum digoxin levels should be closely monitored when a patient begins receiving clarithromycin and other macrolide antibiotics.