Patients with breast cancer, who are treated with curative intent, have a combination of surgery (excision of the tumor) and a course of medical therapy (chemotherapy or endocrine treatment). Both treatments are associated with significant side effects. Chemotherapy is associated with nausea, vomiting, hair loss and bone marrow suppression, whereas endocrine therapy is associated with arthritis and menopausal symptoms. Patients taking either chemotherapy or endocrine treatment may experience a range of side effects. The range and severity of side effects experienced vary from patient to patient. Little or nothing is known about the reason for this difference.