Background: Adherence to adjuvant endocrine therapy (AET) greatly increases the survival rate of patients with estrogen receptor-positive breast cancer. However, the reasons for nonadherence, as well as the associated factors, are not known. The purpose of this study was to investigate whether the factors associating with intentional nonadherence are different from those associating with unintentional nonadherence. Methods: Two hundred ten patients with estrogen receptor-positive breast cancer undergoing AET were enrolled in this cross-sectional study. Adherence to AET was self-reported on the Morisky Medication Adherence Scale-8 (MMAS-8) questionnaire, and each query was scored as either intentional or unintentional. Patients adhering to AET (n = 54, subgroup 1, control) had a MMAS-8 score of 8. By contrast, patients nonadhering to AET were categorized as unintentional (n = 62, subgroup 2), intentional (n = 9), or mixed-type (n = 85, subgroup 3). Information on sociodemographic and clinical variables, including AET beliefs and symptoms of menopause and depression, was also collected and compared among subgroups. Results: Nine variables were evaluated by multivariate logistic regression analysis. An unfavorable balance in AET beliefs and mild or severe symptoms of depression were independent predictors of mixed-type nonadherence vs. adherence [adjusted odds ratio (OR) = 5.89 (95% CI, 1.80�19.25) and 4.30 (95% CI, 1.41�13.09), respectively], but not of unintentional nonadherence vs. adherence (all P > 0.05). Age (<50 years old) was the only independent predictor of unintentional nonadherence vs. adherence [adjusted OR = 3.49 (95% CI, 1.09�11.17)]. Conclusions: The factors associating with intentional nonadherence to AET are different from those associating with unintentional nonadherence. These findings can be applied to clinical protocols to improve adherence to AET by breast cancer patients.

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