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Healthy User Bias in Comparative Safety Studies for Brand‐Name vs. Generic Products: The Example of Warfarin
Author(s) -
Bird Steven T.,
Flowers Natasha,
Zhao Yueqin,
McKean Stephen,
Izem Rima,
Wernecke Michael,
Kozlowski Steven,
MaCurdy Thomas E.,
Kelman Jeffrey A.,
Graham David J.
Publication year - 2019
Publication title -
clinical pharmacology and therapeutics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.941
H-Index - 188
eISSN - 1532-6535
pISSN - 0009-9236
DOI - 10.1002/cpt.1498
Subject(s) - warfarin , medicine , medical prescription , atrial fibrillation , prescription drug , population , pharmacy , confounding , managed care , emergency medicine , family medicine , intensive care medicine , environmental health , health care , pharmacology , economics , economic growth
Warfarin was selected as a case study to examine confounding when comparing a product across different manufacturers because it is a narrow therapeutic index drug with prevalent beliefs for brand‐name superiority. Medicare beneficiaries aged ≥65 years with atrial fibrillation and an incident outpatient warfarin prescription from July 2006 through July 2015 were included in the study population ( N = 746,098). Substantial imbalances were observed between brand‐name warfarin and generics for (i) clinical comorbidity, (ii) socioeconomic status, (iii) prescriber specialty, (iv) recent ambulatory and emergent care, (v) drug adherence, (vi) pharmacy setting (e.g., retail, mail‐order), and (vii) risk scores for bleeding and thrombosis. Patients receiving brand‐name warfarin were healthier than patients receiving generic manufactured warfarin. Utilization of generic warfarin products also differed by geographic region and pharmacy setting. Manufacturer‐level comparative‐safety studies for causal inference should carefully consider the presence of these imbalances and their potential for introducing healthy user bias.