People undergoing treatment for various malignancies often use of dietary supplements during their therapy, perhaps without sufficient regard to the potential consequences. It is possible that some supplements (especially antioxidants), could compromise the effectiveness of chemotherapy. Recently, researchers from various North American centres have collaborated we on a prospective study to evaluate determine if there is any association between supplement use and breast cancer treatment outcomes.1134 Patients with breast cancer who had been randomly assigned to a clinical trial assessing the effectiveness of a relatively standard chemotherapy regimen also provided information about use of supplements. Use of any antioxidant supplement (vitamins A, C, and E; carotenoids; coenzyme Q10) both before and during treatment was associated with an increased hazard of recurrence. Vitamin B12 use both before and during chemotherapy was significantly associated with poorer disease-free survival , as was use of iron. Multivitamin use was not associated with survival outcomes. The study, published in the respected Journal of Clinical Oncology, can be accessed here.