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What is known and objective Unexplained thrombocytopenia is a common clinical problem, and the possibility of drug-induced thrombocytopenia must be considered. Moreover, chemotherapy agents are known to typically cause thrombocytopenia by suppressing haematopoiesis, but they can also cause immune thrombocytopenia in which the mechanism involves antibody-mediated platelet destruction. Case description We report a case of severe thrombocytopenia related to trastuzumab administration in which the patient exhibits a slow drift downward in platelet counts with repeated cycles of trastuzumab. A 70-year-old woman received a diagnosis of breast cancer and was treated with trastuzumab. She was confirmed to have a severe trastuzumab-induced thrombocytopenia after the 4th cycle. Retreatment with trastuzumab was not attempted. What is new and conclusion Considering that trastuzumab is being increasingly used and that severe thrombocytopenia is a potentially life-threatening complication, its association must be seriously considered. [ABSTRACT FROM AUTHOR]