If the Schilling test is also normal, then the cause may be genetic or secondary to other drugs or disease. From Medline:

'Deficiencies or malabsorption of Vitamin B-12 and folic acid are the most common causes of megaloblastic anemia. Other causes are leukemia, myelofibrosis, multiple myeloma, certain hereditary disorders, drugs that affect DNA synthesis such as chemotherapy agents (methotrexate), alcohol, and other causes. Risk factors relate to the causes'.

If all of the above does not apply, and the cause is idiopathic, then you could consider using the larger anemia rubric and see if one of those remedies fits the constitutional picture.