The full coverage policy for essential medicines (FCPEMs) was proposed and implemented in Taizhou city of Zhejiang Province, China, to promote equal access and adherence to medicines. This study aimed to examine the effects of FCPEMs on the income-related inequality in medication adherence among local patients with hypertension or diabetes. We collected electronic health records of patients with hypertension or diabetes of three districts of Taizhou from 2011 to 2016. As the implementation schedule of the FCPEMs varied across districts, we applied a retrospective longitudinal study design and assigned records from 1 year before to 3 years following the implementation of FCPEMs as baseline and follow-up data. We thus generated a dataset with 4-year longitudinal data. The concentration index (CI) and its decomposition method were employed to measure factors contributing to inequality in medication adherence and the role played by FCPEMs. The sample size rose from 264,836 at the baseline to 315,677, 340,512, and 355,676 by each follow-up year, and the proportion of patients taking free medicines rose from 17.6 to 25.0 and 29.8% after FCPEMs implementation. The proportion of patients with high adherence increased from 39.9% at baseline to 51.6, 57.2, and 60.5%, while CI decreased from 0.073 to -0.011, -0.029, and -0.035. The contribution of FCPEMs ranked at 2(nd)/13, 7(th)/13, and 2(nd)/13 after the implementation of FCPEMs. Changes in CI of medication adherence for every 2 years were -0.084, -0.018, and -0.006, and the contribution of FCPEMs was -0.006, 0.006, and 0.007, ranking at 2(nd)/13, 2(nd)/13, and 1(st)/13, respectively. Most changes in CI of medication adherence can be attributed to FCPEMs. The medication adherence of patients with hypertension or diabetes improved after the implementation FCPEMs in Taizhou, although inequality did not improve consistently. In general, FCPEMs could be a protective factor against income-related inequalities in access and adherence to medicines. Future research is needed to investigate the change mechanism and the optimal design of similar interventions.
