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

ObjectivesTo assess the association between multiple pharmacy use and medication adherence and potential drug-drug interactions (DDIs) in older adults. DesignCross-sectional propensity score-weighted analysis. Setting2009 claims data. ParticipantsA 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. MeasurementsMultiple 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. ResultsOverall, 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). ConclusionsFilling 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.