A drug used under the brand names Neupogen, Zarxio, and Granix to treat neutropenia (a condition in which there is a lower-than-normal number of white blood cells) caused by some types of chemotherapy. It is also used under the brand names Neupogen and Zarxio to help prevent infection in patients with certain types of cancer, to treat severe chronic neutropenia, and to prepare the blood for the collection of certain types of blood cells. Filgrastim helps the bone marrow make more white blood cells. It is a type of colony-stimulating factor. Also called G-CSF and granulocyte colony-stimulating factor.