Backgrounds Myelosuppression is decreased bone marrow activity that can be caused by chemotherapy which lowers the formation of blood cells and manifests as anaemia, neutropenia, leukopenia, and thrombocytopenia. Objective This study aims to analyze the post-chemotherapy myelosuppression in breast cancer patients and the chemotherapy regimen used during chemotherapy associated with blood parameters. Method This study was conducted in observational retrospective design. Data were taken from patients’ medical record that went on chemotherapy during January 2021- December 2022. Results The result showed that 97% patients had myelosuppression with the significance statistic’s analysis P<0,05. The chemotherapy regimens used in breast cancer patients are AC-T, TAC, Paclitaxel-Carboplatin, TC, AC-TH, and TCH. This study found myelosuppression in several regimens: anaemia grade I-II in patients with AC-T regimen, neutropenia grade I-IV in patients with TCH regimen, leukopenia grade I-IV in patients with TC regimen, and thrombocytopenia grade I in patients with Paclitaxel-Carboplatin regimen. The incidence of delayed chemotherapy in patients due to myelosuppression is 18%. Conclusion Based on this study, all the regimens used had myelosuppression side effect. The highest incidence of myelosuppression occurred in patient with TAC and TCH regimen. Myelosuppression can cause delayed therapy, dose reduction, and drug substitution.