OBJECTIVES: To understand the proportion of total medical costs attributed to cancer drug treatment in oncology patients, with a focus on breast and prostate cancer.  METHODS: Literature review conducted using PubMed, Medscape and Google to identify and retrieve studies in cancer patients reporting total medical costs and cost components, particularly cancer drug treatment costs. All studies reporting data on US American patients published in English language between 2009 and 2013 were included. RESULTS: A total of 5565 studies were reviewed, 14 of which were relevant and included. Total medical costs and cost components varied substantially by tumor type and location, cancer stage, phase of care, study design, payers and the way drug and treatment costs were defined and reported. In 2011, oncology care costs accounted for $125 billion. Prescription drug costs were 1.8–13.9% of total cancer care costs, with 13.9% representing the proportion of costs during the last six months of life. Studies of breast cancer patients reported that drugs accounted for 2.4–37.0% of total costs, with the lowest proportion occurring in newly diagnosed and the highest in metastatic patients, respectively. Outpatient visits were the primary cost driver, accounting for 64.0-82.0% of total costs. Among patients receiving chemotherapy as their primary treatment, chemotherapy costs accounted for 25.0% and other medications (e.g., analgesics, sedatives, anti-emetics, antidepressants) accounted for 26.0% of total costs. None of the prostate cancer studies identified met the research inclusion criteria.   CONCLUSIONS: Studies on drug treatment costs are limited and results vary substantially depending on patient clinical characteristics, cancer stage, and disease progression. Existing studies suggest that although medical costs are high overall, cancer drug treatment costs only contribute 1.8–13.9% of total cancer costs, and 2.4–37.0% in breast cancer.