Background Medication errors are avoidable occurrences that can assume clinically significant dimensions and impose relevant costs to the health system, especially in the context of antineoplastic therapy. Objective Assess the clinical significance and economic impacts of a clinical pharmaceutical service. This retrospective study consists of an analysis of pharmacy interventions and drug-related problems found in a review of electronic prescriptions referring to antineoplastic therapy of a public teaching tertiary hospital in Brazil. Method Retrospective descriptive study obtained from electronic records of drug-related problems and pharmaceutical interventions related to antineoplastic therapy for oncological and hematological diseases, obtained through the pharmacotherapy review service. The accepted interventions were analyzed for the financial impact generated, evaluating your direct costs. The perception of clinical significance of a random sample of interventions was ascertained by the experts' opinion, using the Delphi method. Results The most frequent problem was a “lack of information to professionals” (25.06%), “problems as to the frequency and interval of doses” (22.90%), and “medication underdosing” (16.20%). Dose adjustment (31.50%) and clarifications (30.90%) were the most frequent pharmaceutical interventions. In the second round of Delphi, experts rated 77.77% of interventions as extremely significant and very significant. The main drugs reported in the interventions were cyclophosphamide, carboplatin, methotrexate, folinic acid, and monoclonal antibodies. The savings amounted to US$1,193,970.18 and involved mainly bortezomib, dactinomycin, and monoclonal antibodies. Conclusion Clinical pharmacy services contributed to the rational use of medicines presenting clinical significance and avoiding waste of financial resources.