Anemia means an abnormally low number of red cells in the blood. Anemia can be caused by:
- inherited genetic conditions
- chronic disease such as kidney disease or certain cancers
- medications such as ribavirin, Dilantin (phenytoin), or methotrexate
- Vitamin B deficiency
- iron deficiency
- losing a lot of blood
- too many red blood cells being destroyed by the body (called hemolytic anemia)
- not enough red cells being produced by the body in conditions such as aplastic anemia or thalassemia
- chemotherapy or radiation therapy
- weakness and fatigue
- pale skin
- shortness of breath with physical activity
- lightheadedness or dizziness
- rapid heartbeat
- headache
Source:
"Anemia." Health Topics A-Z. 01 Jan 2008. A.D.A.M.. 9 Mar 2009.

