Snapshot A 45-year-old woman undergoing chemotherapy for breast cancer undergoes routine laboratory work-up. A complete blood count is significant for neutropenia and anemia. She has been on filgrastim for chemotherapy-related neutropenia. She follows up with her medical oncologist, who suggests switching to sargramostim, which works in a slightly different mechanism. Introduction Drugs filgrastim recombinant human granulocyte-colony stimulating factor (G-CSF) pegfilgrastim recombinant PEG-conjugated human G-CSF sargramostim recombinant granulocyte macrophage CSF (GM-CSF) Mechanism of action G-CSF stimulates production of neutrophils GM-CSF stimulates production of neutrophils, dendritic cells, and monocytes/macrophages Clinical use neutropenia myelosuppressive chemotherapy aplastic anemia congenital neutropenia stem cell transplantation Adverse effects nausea and vomiting bone pain cutaneous vasculitis allergic reaction angioedema urticaria anaphylaxis thrombocytopenia capillary leak syndrome leukocytosis