Lay summary
Approximately 70 percent of women with localised breast cancer receive adjuvant systemic treatment including chemotherapy, biological treatments and endocrine therapy. The aim of these treatments is to reduce the risk of cancer relapse following surgery. Patients diagnosed with breast cancer are older and are likely to have comorbidities resulting in polypharmacy. Systemic therapy can cause significant side effects and can have drug-drug interactions with other medications causing toxicity or reduced effectiveness. This raises a number of questions for patients receiving additional medications for comorbid conditions. Do the harms of adjuvant systemic therapy outweigh the benefits? How do the impacts vary by different treatments and cancer stage? This project aims to examine whether polypharmacy affects the systemic treatment and outcomes of breast cancer patients. It will provide essential evidence for clinicians and patients on their decision-making as to which patients with existing co-morbidities are most likely to benefit from systemic anti-cancer treatment.