BACKGROUND: Despite the questionable effectiveness of oral complementary and alternative medicine (OCAM) in relieving cancer-related symptoms, including fatigue (CRF), many patients use it aiming to improve their quality of life. We assessed factors associated with OCAM use, focusing on CRF. METHODS: Women with stage I-III breast cancer (BC) were included from CANTO (NCT01993498). OCAM use was defined as taking homeopathy, vitamins/minerals, or herbal/dietary supplements. Multivariable multinomial logistic regressions evaluated associations of CRF (EORTC QLQ-C30), patient, and treatment characteristics with OCAM use. RESULTS: Among 5237 women, 23.0% reported OCAM use overall (49.3% at diagnosis, 50.7% starting post-diagnosis), mostly homeopathy (65.4%). Mean (SD) CRF score was 27.6 (24.0) at diagnosis and 35.1 (25.3) at post-diagnosis. More intense CRF was consistently associated with OCAM use at diagnosis and post-diagnosis [adjusted odds ratio (aOR) for 10-point increase 1.05 (95% Confidence interval 1.01-1.09) and 1.04 (1.01-1.09) vs. never use, respectively]. Odds of using OCAM at diagnosis were higher among older [for 5-year increase, 1.09 (1.04-1.14)] and more educated patients [college vs. primary 1.80 (1.27-2.55)]. Women with income > 3000 [vs.