INTRODUCTION: Despite evidence of the efficacy of adjuvant endocrine therapy (AET) in reducing the risk of recurrence and mortality after treatment for primary breast cancer, adherence to AET is suboptimal. This study aimed to explore factors that influence adherence and nonadherence to AET following breast cancer to inform the development of supportive interventions. METHODS: Interviews were conducted with 32 women who had been prescribed AET, 2-4 years following their diagnosis of breast cancer. Both adherers (n=19) and nonadherers (n=13) were recruited. The analysis was conducted using the Framework approach. RESULTS: Factors associated with adherence were as follows: managing side effects including information and advice on side effects and taking control of side effects, supportive relationships, and personal influences. Factors associated with nonadherence were as follows: burden of side effects, feeling unsupported, concerns about long-term AET use, regaining normality, including valuing the quality of life over length of life, and risk perception. CONCLUSION: Provision of timely information to prepare women for the potential side effects of AET and education on medication management strategies are needed, including provision of timely and accurate information on the efficacy of AET in reducing breast cancer recurrence and on potential side effects and ways to manage these should they arise. Trust in the doctor-patient relationship and clear patient pathways for bothersome side effects and concerns with AET are important. Training and education on AET for GPs should be considered alongside novel care pathways such as primary care nurse cancer care review and community pharmacist follow-up.