A drug used 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 used to help prevent infection in patients with certain types of cancer. It is also used to treat chronic neutropenia and to prepare the blood for the collection of certain types of blood cells. Neupogen is also used to help prevent damage to the bone marrow in patients who were exposed to very high doses of certain types of radiation. Neupogen helps the body make more white blood cells. It is a type of colony-stimulating factor. Also called filgrastim, G-CSF, granulocyte colony-stimulating factor, and Zarxio.