Effect of Multiple Pharmacy Use on Medication Adherence and Drug–Drug Interactions in Older Adults with Medicare Part D

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Abstract
Objectives: To assess the association between multiple pharmacy use and medication adherence and potential drug–drug interactions ( DDI s) 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 DDI s 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 DDI s in concurrent pharmacy users.
Funding Information
  • National Institute on Aging (P30 AG024827, K07AG033174, R01AG027017)
  • Agency for Healthcare Research and Quality (R01HS018721)
  • Veterans Affairs Health Services Research and Development Service Career Development Award (CDA 09-207)

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