Effect of multiple pharmacy use on medication adherence and drug-drug interactions in older adults with Medicare Part D.

Objectives: To assess the association between multiple pharmacy use and medication adherence and potential drug-drug interactions (DDIs) in older adults.
Design: Cross-sectional propensity score-weighted analysis.
Setting: 2009 claims data.
Participants: A nationally representative sample of 926,956 Medicare Part D beneficiaries aged 65 and older continuously enrolled in fee-for-service Medicare and Part D that year who filled one or more prescriptions at a community retail or mail order pharmacy.
Measurements: Multiple pharmacy use was defined as concurrent (overlapping time periods) or sequential use (non-overlapping time periods) of ≥ 2 pharmacies in the year. Medication adherence was calculated using a proportion of days covered of 0.80 or greater for eight therapeutic categories (beta-blockers, renin angiotensin system antagonists, calcium channel blockers, statins, sulfonylureas, biguanides (metformin), thiazolidinediones, and dipeptidyl peptidase-IV inhibitors). Potential DDIs arising from use of certain drugs across a broad set of classes were defined as the concurrent filling of two interacting drugs.
Results: Overall, 38.1% of the sample used multiple pharmacies. Those using multiple pharmacies (concurrently or sequentially) consistently had higher adjusted odds of nonadherence (ranging from 1.10 to 1.31, P < .001) across all chronic medication classes assessed after controlling for sociodemographic, health status, and access to care factors than single pharmacy users. The adjusted predicted probability of exposure to a DDI was also slightly higher for those using multiple pharmacies concurrently (3.6%) than for single pharmacy users (3.2%, adjusted odds ratio (AOR) = 1.11, 95% confidence interval (CI) = 1.08-1.15) but lower in individuals using multiple pharmacies sequentially (2.8%, AOR = 0.85, 95% CI = 0.81-0.91).
Conclusions: Filling prescriptions at multiple pharmacies was associated with lower medication adherence across multiple chronic medications and a small but statistically significant greater likelihood of DDIs in concurrent pharmacy users.
(© 2014, Copyright the Authors Journal compilation © 2014, The American Geriatrics Society.)