Metronomic cyclophosphamide is occasionally used as maintenance therapy for patients with metastatic triple-negative breast cancer (TNBC), although there is limited long-term safety data. In this case study, we present the patient with metastatic TNBC who achieved a durable complete response and subsequently received metronomic cyclophosphamide for seven years. Progressive cytopenias prompted an investigation, which revealed myelodysplastic syndrome with monosomy 7 once reversible causes had been addressed, including vitamin deficiencies, Helicobacter pylori infection and previous hepatitis C exposure. Following discontinuation of cyclophosphamide, gradual haematological recovery was observed, and the patient remains without radiological evidence of progression. This case highlights the potential for late haematological toxicity associated with prolonged exposure to low doses of alkylating agents, emphasises the need for periodic reassessment of maintenance therapy in long-term responders, and shows that persistent cytopenias require evaluation of alternative causes rather than being solely attributed to chemotherapy.