title: Warfarin-associated bleeding events and concomitant use of potentially interacting medicines reported to the Norwegian spontaneous reporting system.
abstract: OBJECTIVE
  To study warfarin associated bleeding events reported to the Norwegian spontaneous reporting system and evaluate the differences in assessment of potentially interacting medicines between reporters and evaluators.
  
  
  METHODS
  Data on bleeding events on warfarin were retrieved from the Norwegian spontaneous reporting system database. Key measurements were time to bleeding, use of concomitant medications and the evaluation done by reporters.
  
  
  RESULTS
  In 289 case reports a total of 1261 medicines (median 4.0 per patient, range 1-17) was used. The evaluators (authors of this article) identified 546 medicines including warfarin (median 2.0 per patient, range 1-7) that could possibly cause bleeding alone or in combination. Reporters assessed 349 medicines (median 1.0 per patient, range 1-4) as suspect. Evaluators identified 156 pharmacokinetic and 101 pharmacodynamic interactions, compared with 19 pharmacokinetic and 56 pharmacodynamic interactions reported as suspected by the reporters. Time to bleeding was stated in 224 reports. Among the early bleeding events, the reports on warfarin without interacting medicines showed the highest INR (international normalized ratio). Heparin was used in 17/21 reported bleeding events during the first week on warfarin. Among the late bleeding events, reports with pharmacokinetic interacting medicines had the highest INR.
  
  
  CONCLUSIONS
  Concomitant use of potentially interacting medicines was involved in the majority of the warfarin-associated bleeding events reported to the Norwegian spontaneous reporting system. Reporters assessed mostly warfarin as the only contributor to bleeding. In particular, pharmacokinetically interacting medicines were not suspected as contributing to bleeding.
authors.affiliation: Department of Pharmacology, Oslo University Hospital, Oslo, Norway. sigrid.narum@studmed.uio.no
authors.forename: Sigrid
authors.initials: S
authors.lastname: Narum
authors.forename: Vigdis
authors.initials: V
authors.lastname: Solhaug
authors.forename: Kirsten
authors.initials: K
authors.lastname: Myhr
authors.forename: Per Wiik
authors.initials: PW
authors.lastname: Johansen
authors.forename: Odd
authors.initials: O
authors.lastname: Brørs
authors.forename: Marianne Kristiansen
authors.initials: MK
authors.lastname: Kringen
mesh_terms: D000328:Adult; D016907:Adverse Drug Reaction Reporting Systems; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D000925:Anticoagulants / Q000008:administration & dosage / Q000009:adverse effects*; D004334:Drug Administration Schedule; D004347:Drug Interactions; D005260:Female; D006470:Hemorrhage / Q000139:chemically induced* / Q000453:epidemiology; D006801:Humans; D019934:International Normalized Ratio; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects*
chemical_list: D000925:Anticoagulants; D014859:Warfarin