Objectives: We evaluated changes in oral diabetes mellitus medication adherence and persistence, as well as glycemic control for the year prior to breast cancer (BC) diagnosis (Year -1), during BC treatment, and in subsequent years.

Methods: Cohort study of 4216 women diagnosed with incident early stage (I and II) invasive BC from 1990-2008, enrolled in Group Health Cooperative. Adherence was measured in prevalent users at baseline (N = 509), during treatment, and 1-3 years post-diagnosis using medication possession ratio (MPR), % adherent (MPR ≥0.80) and discontinuation rates. Laboratory data on glycosylated hemoglobin (HbA1c ) was obtained for the corresponding periods.

Results: Compared with Year -1, mean MPR for metformin/sulfonylureas (0.86 vs 0.49, p < 0.001) and % adherent (75.3% vs 24.6%, p < 0.001) declined during BC treatment. MPR and % adherent rose slightly during Years 1-3 post-diagnosis but never returned to baseline. Discontinuation rates increased from treatment to Year +1 (59.3% vs 75.6%, p < 0.001) and remained elevated during subsequent observation periods. Compared with baseline, increased HbA1c (7.0% vs 7.4%, p = 0.001) and % women with high HbA1c >7.0% (34.9% vs 51.1%, p < 0.001) coincided with decreased adherence.

Conclusion: Diabetes mellitus medication adherence declined following BC diagnosis, whereas discontinuation rates were relatively stable but poor overall. The proportion of adherent users increased only marginally following treatment, whereas the proportion of women meeting goals for HbA1c decreased considerably. These data support the hypothesis that adherence and subsequent glycemic control are sensitive to BC diagnosis and treatment. Confirmatory studies in other settings, on reasons for reduced adherence post-cancer diagnosis, and on subsequent indicators of glycemic control are warranted.