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weeks after the onset of osimertinib administration, with a bone marrow biopsy yielding a diagnosis of severe aplastic anemia. Symptoms of drug-induced aplastic anemia have been found to develop from days to months after initiation of treatment with the offending drug. 4 The present patient had taken L. bifidus, ethyl icosapentate, eldecalcitol, lansoprazole, amlodipine, and sitagliptin phosphate hydrate for more than 2 years, with none of these agents having previously been associated with aplastic anemia. Osimertinib was therefore most likely the cause of aplastic anemia in the proband, although other causes cannot be completely ruled out. As far as we are aware, this would be the first report of a case of osimertinib-induced severe aplastic anemia.Aplastic anemia is a rare hematopoietic stem cell disorder that results in pancytopenia and hypocellularity of bone marrow and can be acquired, hereditary, or idiopathic. Acquired aplastic anemia has been causally associated with many agents including drugs. The three major causes of acquired aplastic anemia are direct toxicity, metabolitedriven toxicity, and immune-mediated mechanisms, 5 with the pathogenesis of osimertinib-associated aplastic anemia being unknown. Other EGFR TKIs have not been found to induce irreversible bone marrow failure, suggesting that EGFR inhibition itself is not a cause of aplastic anemia.In conclusion, physicians should be aware of the risk for aplastic anemia in patients treated with osimertinib. Such patients should therefore be monitored carefully, and the frequency of this severe adverse effect should be determined.

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