Depletion of the body's stores of iron is usually a result of chronic, hidden bleeding. The lack of a chemical called intrinsic factor renders the body unable to absorb the vitamin B 12 needed to produce red blood cells. Intrinsic factor is manufactured in the lining of the stomach. Supplies can be disrupted by certain parasites, inflammatory bowel disease, and diseases of the small intestine. Disorders such as thyroiditis and diabetes can also be at fault.
Alcoholism is another potential culprit.
Anything that harms bone marrow can lead to a decline in blood cell production. Immunosuppressive drugs, anticancer drugs, and other toxic chemicals bring on many cases in this category. Destruction of red blood cells can follow the use of certain drugs, mismatched blood transfusions that cause antibodies to attack red blood cells, or a drug allergy. Some cases of this disorder are inherited. Other symptoms vary according to the underlying cause.