In patients with advanced or metastatic breast cancer (mBC), adherence to treatment, supportive therapy, and comorbidity medication is important for prolonging survival time, reducing symptom burden and side effects, and improving quality of life. The aim of the present study was to determine medication adherence and potential influencing factors in patients with advanced or mBC.

Adults with advanced or mBC treated at the University Hospital Würzburg living in Bavaria, were included in the BRE-BY-MED “Breast Cancer Care in Bavaria for Patients with Metastatic Disease” cohort study (DRKS00026601). Self-reported adherence was analysed using data from patients at baseline, and after 3-, 6-, and 12-months of follow-up, and using Bavarian cancer registry data.

Between July 2022 and February 2024, 93 patients (median age 57 years; IQR = 48–64), were consecutively enrolled in the BRE-BY-MED study. At baseline, 75.3% (n = 70) and 86.0% (n = 80) of the patients reported being adherent and dosage adherent, respectively. During the follow-up period, no relevant changes in adherence levels were observed. Supportive therapy and medication for comorbidities were most affected by non-adherence. The most common reasons for non-adherence were medication beliefs, forgetfulness, and side effects. For 81.2% (n = 69) of the patients, complementary cancer registry data were available and complete, and therapy persistence was found in 78.3% (n = 54) of these patients.

The observed adherence rates indicate that tailored interventions to improve adherence to supportive therapy and medication for comorbidities in patients with advanced or mBC are needed, particularly considering the reasons for non-adherence.