OBJECTIVES: To generate costs and costing algorithms for treatment and supportive drugs in oncology using provincial (Ontario) administrative databases. METHODS: A cohort of women diagnosed with breast cancer (BC) (ICD-9 174.x) was identified from the Ontario Cancer Registry (2007-2010). Firstly, the Ontario Drug Benefit Formulary (ODBF), New Drug Funding Program (NDFP) and Activity Level Reporting (ALR) databases was used in which BC-specific treatments (chemotherapies and hormonal therapies) and supportive drugs were identified. Secondly, unit costs were applied to calculate the overall and per drug costs in each database. Lastly, costing algorithms were generated to conduct the costing analyses.   RESULTS: We identified 30,338 women diagnosed with BC. All chemotherapies and hormonal therapies were named as well as anti-nausea, pain (opioid and non-opioid), anti-infectives, and blood products for supportive drugs. Outputs include number of patient cases with at least one treatment or supportive drug being utilized and total costs. Preliminary results for the 20,076 BC cases prescribed a drug in ODBF totalled $69.5 million in which $37.5 million was treatment-specific. CONCLUSIONS: We have generated preliminary ODBF costs for oncology drugs in BC and costs for the NDFP and ALR databases will be determined next. These costing algorithms will allow for the calculation of oncology treatment and supportive drug costs in different cancer cohorts.