There are numerous causes of anemia. Acute anemia is most commonly due to blood loss or to destruction of the red blood cells. The source of blood loss may or may not be obvious; hemorrhage can occur due to an external wound, for example, but internal bleeding is also possible. Sources of internal bleeding include hemorrhage into the lungs, joints, GI tract, and body cavities (abdomen is most common). Alternatively, red blood cells can be destroyed secondary to immune-mediated, toxic, infectious, or cancerous processes.

Chronic anemia may be from iron-deficiency, chronic or inflammatory disease, bone marrow dysfunction, or kidney disease, among other causes. Chronic anemia may be better tolerated until it becomes severe because the body has had time to adjust to the lower red blood cell count. With severe anemia, however, the body cannot acclimate regardless of chronicity.