The validity of diagnostic vitamin B12 or folic acid blood assays could be compromised by medications, and this should be considered before relying on such tests for therapy. Vitamin B12 is not a substitute for folic acid and since it might improve folic acid deficient megaloblastic anemia, indiscriminate use of vitamin B12 could mask the true diagnosis. Hypokalemia and thrombocytosis could occur upon conversion of severe megaloblastic to normal erythropoiesis with B12 therapy. Therefore, serum potassium levels and the platelet count should be monitored carefully during therapy. Vitamin B12 deficiency may suppress the signs of polycythemia vera. Treatment with vitamin...