BACKGROUND: Nonadherence to medication is a prevalent issue among patients with type 2 diabetes mellitus (T2DM). The family doctor system promotes continuous, integrated, and personalized primary care, which may improve medication adherence. However, more evidence is needed regarding causal association between family doctor system and medication adherence. This study is to assess the impact of the family doctor system on medication adherence among T2DM patients. METHODS: This cohort study utilized data between 2014 and 2019 from three separate administrative databases of an eastern city in China. Adult patients with T2DM who continuously registered with family doctors from 2015 to 2019 (n = 18,841) were assigned to the intervention group, while those who never registered during this period (n = 1,429) were classified as the control group. A difference-in-differences design was employed to compare medication adherence between registered and unregistered T2DM patients after the first stage of family doctor system in 2015 and the second stage in 2018. Propensity score matching was applied to ensure the robustness. The mean proportion of days covered (PDC), and proportion of patients with good adherence (PDC ≥ 80%) were the outcomes of interest. All recommended T2DM medications were included to calculate PDC. RESULTS: Compared to the unregistered group, PDC across all institutions for registered patients increased by 5.0% (p <
0.001) after the first stage of family doctor system, and by 5.9% (p <
0.001) after the second stage. The proportion of patients with good adherence increased by 9.5% (p <
0.001) and by 11.8% (p <
0.001) after two stages, respectively. The adherence improvement was more pronounced in community health centers. However, the overall good adherence rate among registered patients in 2019 remained relatively low, reaching only 59.77%. CONCLUSIONS: The family doctor system significantly improved medication adherence among T2DM patients by providing patient-centered, continuous, and integrated primary services, especially in community health centers. Nevertheless, further efforts should be made to enhance medication adherence.