Hematologic dysfunction, including thrombocytopenia, anemia, neutropenia, thromboses, and coagulopathy, occur commonly during critical illnesses. A major challenge is to identify drug-induced causes of hematologic dysfunction. Given the wide variety of drug-induced hematologic effects, clinicians always should consider any concomitant drugs in the differential diagnosis of acquired hematologic dysfunction. The most severe effects include drug-induced aplastic anemia, heparin-induced thrombocytopenia, and drug-induced thrombotic microangiopathy. Certain drugs are associated with multiple hematologic effects. For example, cisplatin can cause hemolytic uremia syndrome and erythropoietin deficiency, and quinine can precipitate immune-mediated thrombocytopenia, immune-mediated thrombocytopenia, and thrombotic microangiopathy.