Adherence and Compliance with Endocrine Treatment After Primary Breast Cancer Treatment: A Cross-Sectional Qualitative Study

Background and Objectives: Breast cancer is the most common cancer in women, with approximately 80% being oestrogen receptor positive, necessitating adjuvant endocrine therapy (AET) to reduce recurrence. Treatment adherence is crucial, yet 10–50% of patients take incorrect doses or discontinue therapy, which is associated with a 20% increase in mortality. AET may also impact bone health. This study aimed to explore patients’ beliefs about endocrine treatment, investigate how perceptions of medication risk and benefit affect adherence, and assess changes in bone mineral density (BMD) during therapy. Materials and Methods: A cross-sectional mixed-method study was conducted. One hundred patients diagnosed with oestrogen receptor-positive breast cancer in 2020 were sent the Beliefs about Medicines Questionnaire–Adjuvant Endocrine Therapy (BMQ-AET) and 101 semi-structured telephone interviews were completed. Initial and most recent Dual-Energy X-ray Absorptiometry (DEXA) scans were compared to assess changes in BMD. Results: The questionnaire response rate was 55% (n = 55). Forty-nine patients returned the postal paper survey and six patients responded via QR code. One hundred and one patients participated in semi-structured telephone interviews. Of the total cohort, 91.7% were adherent to AET, while 13 patients (8.3%) were non-adherent. Non-adherent patients had significantly lower BMQ-AET Necessity scores (mean 12.08 vs. 19.22; median 12 vs. 20; p < 0.001) and higher Concerns scores (mean 17 vs. 13.46; Median 17 vs. 13; p = 0.002). The most common reasons for non-adherence were joint pain and reduced quality of life (58%), highlighting a need for additional support in managing side effects. Among the participants with suitable DEXA data, the majority (54.2%) demonstrated an increase in BMD over time. Conclusions: This study demonstrates high adherence to AET, with non-adherent patients showing lower perceived necessity and greater concern about treatment. These findings emphasise the importance of addressing patient beliefs to enhance adherence. The observed improvements in BMD suggest that proactive bone health management, alongside AET, may mitigate expected declines, challenging conventional assumptions regarding therapy-related bone loss.