title: Potential drug-drug interactions in the outpatient setting.
abstract: BACKGROUND
  Although medication safety research has tended to focus on inpatients, the safety of drug use among outpatients is also a concern.
  
  
  OBJECTIVE
  We estimate the frequency of potentially interacting concomitant medication dispensing among outpatients.
  
  
  METHODS
  We report the number and percent of patients annually dispensed an object drug of interest (ie, warfarin, digoxin, cyclosporine, or lovastatin/simvastatin) with a potentially interacting drug among a random sample of insured adults receiving care from 10 integrated delivery systems. We use 2 definitions of concomitant dispensing: medications dispensed: 1) during the time period for which the patient had the other medication available ('days supply') and 2) on the same day. We also estimate the number of insured U.S. population codispensed these medication pairs.
  
  
  RESULTS
  Among patients dispensed a drug of interest, between 17.8% (95% confidence interval [CI]=17.1-18.6%) and 28.0% (95% CI=24.0-32.1%) were concomitantly dispensed a potentially interacting drug using the "days supply" definition, and between 7.1% (95% CI=6.6-7.7%) and 17.7% (95% CI=14.4-21.1%) using the "same day" definition. Extrapolating to the insured U.S. population, between 1.29 (95% CI=1.25-1.33; same day) and 2.67 (95% CI=2.62-2.72; days supply) million insured adults are dispensed 1 of these 4 potentially interacting pairs.
  
  
  CONCLUSIONS
  We found evidence of potentially interacting concomitant medication dispensing among outpatients. An opportunity exists to better understand how such dispensing translates into adverse events and ultimately to improved medication safety.
authors.affiliation: Henry Ford Health System, Detroit, Michigan 48202, Channing Laboratory, Brigham and Women's Hospital, and Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, USA. jlafata1@hfhs.org
authors.forename: Jennifer Elston
authors.initials: JE
authors.lastname: Lafata
authors.forename: Lonni
authors.initials: L
authors.lastname: Schultz
authors.forename: Jan
authors.initials: J
authors.lastname: Simpkins
authors.forename: K Arnold
authors.initials: KA
authors.lastname: Chan
authors.forename: John R
authors.initials: JR
authors.lastname: Horn
authors.forename: Scott
authors.initials: S
authors.lastname: Kaatz
authors.forename: Charron
authors.initials: C
authors.lastname: Long
authors.forename: Richard
authors.initials: R
authors.lastname: Platt
authors.forename: Marsha A
authors.initials: MA
authors.lastname: Raebel
authors.forename: David H
authors.initials: DH
authors.lastname: Smith
authors.forename: Huago
authors.initials: H
authors.lastname: Xi
authors.forename: Marianne Ulcickas
authors.initials: MU
authors.lastname: Yood
mesh_terms: D000368:Aged; D004347:Drug Interactions; D004363:Drug Utilization; D064420:Drug-Related Side Effects and Adverse Reactions; D005260:Female; D006279:Health Maintenance Organizations; D006801:Humans; D019458:Insurance Coverage / Q000706:statistics & numerical data; D007348:Insurance, Health / Q000706:statistics & numerical data; D008297:Male; D008875:Middle Aged; D010045:Outpatients / Q000706:statistics & numerical data*; D019338:Polypharmacy