OBJECTIVES Hepatitis C virus treatment is based on the use of Pegylated or Convencional Interferon combined with Ribavirin according to HCV genotype in Brazil. During this treatment usually it is observed adverse events that may need additional medications leading to increased demand on health resources.The aim of this study was to evaluate the impact of the costs of the additional drugs used by chronic HCV patients during the treatment with Pegylated or Convencional Interferons combined with Ribavirin. METHODS It was used a retrospective cohort of HCV patients receiving medication from the Pharmacy Division and followed at the Clinical Hepatology Unit from the HC-FMUSP. Using the SIGHTM (Sistema de Informação e Gestão Hospitalar) – PRODESP (Tecnologia da Informação), 117 patients were identified as receiving treament for chronic HCV during the period of June 2005 and August 2007. Costs were determined using the Sistema de Administração de Materiais (SAM) from the HC-FMUSP and were compared to Câmara de Regulação do Mercado de Medicamentos, ABC-Farma e DATASUS. Comercial USD = R$2.31. RESULTS The total cost of convencional interferon 3 million UI was US$12,736.76, peginterferon 2a 180mcg US$168,739.74 and peginterferon 2b 80mcg US$223,325.09. The concomitant medications were US$33,806.66 using the price from the SAM, while the value from the CMED would be US$557,572.16. The total cost of medications used by these patients was US$433,608.26, and 7.71% was due to concomitant medications. CONCLUSIONS There is a great difference among the source of prices to evaluate cost of treatment and concomitant medications should be considered in evaluation of chronic hepatitis C patients.