Stem cells donors take G-CSF for 5 to 6 days before donating. Besides increasing the number of stem cells and white blood cells in the bloodstream, the drug also causes some other temporary changes in blood chemistry. Many people who take G-CSF for a few days get a headache, feel an ache in their bones, or feel tired for a few days. About one-third of patients chronically treated with G-CSF to raise blood cell counts develop an enlarged spleen. It is not known if a brief 5- to 6-day course of G-CSF also affects the size of the spleen, but about 1 in 10,000 blood stem cell donors have had a spontaneous rupture of the spleen.