Fatigue is one of the most prevalent and distressing symptoms experienced by cancer patients. Treatment options for cancer-related fatigue are limited; additional therapies are a high priority for research. Bupropion has been identified as a potential therapy for cancer-related fatigue since 1999. Bupropion has diverse actions that target pathways associated with cancer-related fatigue, including inflammation and hypothalamic pituitary adrenal (HPA) axis functioning. Metabolism of bupropion by cytochrome P 450 B6 (CYP2B6) has been extensively characterized. Two pilot studies of bupropion have shown promise in reducing cancer-related fatigue but to date no adequately-powered, randomized controlled trials have been conducted. The goal of the current study is to conduct the first adequately-powered, randomized, double-blinded, placebo-controlled trial of bupropion for cancer-related fatigue. Because fatigue is well-characterized in women with breast cancer, the study will focus on this population. A sample of 422 disease-free breast cancer patients who completed chemotherapy and/or radiotherapy 12-60 months previously and report significant fatigue will be recruited through the University of Rochester Cancer Center (URCC) National Cancer Institute Community Oncology Research Program (NCORP). Participants will be randomized 1:1 to receive generic bupropion XL or placebo. Fatigue will be assessed prior to randomization and 12 weeks later. Blood and saliva will be collected to measure bupropion metabolites, inflammation, CYP2B6 genotype, and cortisol (a marker for HPA axis functioning). Data will be used to address the following aims: 1) to determine the efficacy of bupropion versus placebo in reducing fatigue in a double-blinded, placebo-controlled, randomized clinical trial of breast cancer survivors with fatigue; 2) to assess the tolerability of bupropion in breast cancer survivors with fatigue, 3) to explore the effects of bupropion on putative mechanisms of cancer-related fatigue, and 4) to explore associations of CYP2B6 genotype with bupropion metabolism and changes in fatigue. Positive results from the current study could revolutionize the treatment of cancer-related fatigue, as bupropion is safe, inexpensive, widely-available, and may be more tolerable and acceptable for many patients than current, limited treatment options. Exploration of the effects of bupropion on putative mechanisms of cancer-related fatigue could open new avenues for additional efficacious treatments. Exploration of genetic moderators of efficacy could help identify which patients are most likely to benefit and tailor dosage.