Potential drug-drug interactions in the outpatient setting.
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.
Lafata
Jennifer Elston
JE
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
Schultz
Lonni
L
Simpkins
Jan
J
Chan
K Arnold
KA
Horn
John R
JR
Kaatz
Scott
S
Long
Charron
C
Platt
Richard
R
Raebel
Marsha A
MA
Smith
David H
DH
Xi
Huago
H
Yood
Marianne Ulcickas
MU
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