Erythropoiesis-stimulating factors (ESAs) such as erythropoetin and its synthetic alternatives (darbopoetin, erythropoetin alpha) are essential hormones that control red blood cell production, and are mainly used in the treatment of anemia. The U.S. Food and Drug Administration has issued several warnings related to the use of ESAs; related concerns included risk of increased tumor growth, decreased survival time and cardiovascular side effects. The effects of erythropoietin on fatigue and cognitive impairment have been examined in patients with cancer in the setting of anemia.