title: Concomitant digoxin toxicity and warfarin interaction in a patient receiving clarithromycin.
abstract: OBJECTIVE
  To report a case of a clarithromycin-associated warfarin interaction and digoxin toxicity in a patient.
  
  
  METHODS
  A 72-year-old white woman with chronic atrial fibrillation receiving long-standing therapy with digoxin 0.25 mg/d and warfarin 22.5 mg/wk was prescribed clarithromycin 500 mg three times daily for eradication of Helicobacter pylori. The patient presented to the emergency department with gastrointestinal symptoms, weakness, dizziness, and visual changes 12 days after initiation of clarithromycin. Laboratory results revealed a serum digoxin concentration of 4.6 ng/mL (normal 1.0-2.6) and an international normalized ratio of 7.3 (2.0-3.0). Digoxin, warfarin, and clarithromycin were discontinued and the patient was admitted to the hospital for treatment to resolve the symptoms and to return laboratory values to a safe range. Reduced dosages of digoxin (0.125 mg/d) and warfarin (17.5 mg/wk) were restarted on day 7 of hospitalization. The patient was discharged on day 11 in good condition.
  
  
  CONCLUSIONS
  Several reports of clarithromycin-induced drug interactions with digoxin and with warfarin have been published. Previously, case reports of macrolide-associated interactions mainly involved erythromycin, but more recently have implicated clarithromycin. The interaction between clarithromycin and warfarin is thought to occur from an inhibition of the cytochrome P450 drug metabolizing system. Clarithromycin is thought to cause digoxin toxicity by an alteration of the digoxin-metabolizing gut flora, thereby causing an increase in the digoxin concentration in susceptible individuals. Drug interactions can occur by different mechanisms in the same patient.
  
  
  CONCLUSIONS
  Potential drug interactions can occur between commonly prescribed medications. It is important to monitor patients for symptoms and alterations in laboratory values to prevent not only serious complications, but also unnecessary hospitalizations.
authors.affiliation: University of Western Ontario, London, Canada.
authors.forename: M J
authors.initials: MJ
authors.lastname: Gooderham
authors.forename: P
authors.initials: P
authors.lastname: Bolli
authors.forename: P G
authors.initials: PG
authors.lastname: Fernandez
mesh_terms: D000368:Aged; D000787:Angina Pectoris / Q000150:complications / Q000188:drug therapy; D000900:Anti-Bacterial Agents / Q000009:adverse effects*; D000925:Anticoagulants / Q000009:adverse effects*; D001249:Asthma / Q000150:complications / Q000188:drug therapy; D001281:Atrial Fibrillation / Q000150:complications / Q000188:drug therapy; D002316:Cardiotonic Agents / Q000009:adverse effects*; D017291:Clarithromycin / Q000009:adverse effects*; D004077:Digoxin / Q000009:adverse effects*; D004347:Drug Interactions; D005260:Female; D006333:Heart Failure / Q000150:complications / Q000188:drug therapy; D016481:Helicobacter Infections / Q000188:drug therapy; D016480:Helicobacter pylori; D006801:Humans; D006973:Hypertension / Q000150:complications / Q000188:drug therapy; D014859:Warfarin / Q000009:adverse effects*
chemical_list: D000900:Anti-Bacterial Agents; D000925:Anticoagulants; D002316:Cardiotonic Agents; D014859:Warfarin; D004077:Digoxin; D017291:Clarithromycin