It is currently well established that supraphysiologic doses of folic acid may reverse the hematologic abnormality in patients who are deficient in vitamin B12.1-4 On the other hand, the effect of large doses of cyanocobalamin in folic acid-deficient patients has not been clearly determined. Although responses to cyanocobalamin therapy have occasionally been observed in megaloblastic anemia associated with infancy,5A,5B pregnancy6A and the puerperium,6A,6B cirrhosis of the liver,7,8 intestinal malabsorption,9 and the ingestion of anticonvulsant drugs,10 they have often been suboptimal or inadequate, whereas folic acid therapy induces prompt remission. Interpretation of these results have been complicated by a number of factors, including: (1) lack of reliable laboratory measures of deficiency of vitamin B12 and folic acid, (2) difficulty in control of dietary folic acid intake, especially in the light of recent evidence indicating the therapeutic efficacy of 25μg. and 50μg.
ZALUSKY R, HERBERT V, CASTLE WB. Cyanocobalamin Therapy Effect in Folic Acid Deficiency. Arch Intern Med. 1962;109(5):545-554. doi:10.1001/archinte.1962.03620170043007