Folic acid should not be given alone or in conjunction with inadequate amounts of hydroxocobalamin for the treatment of megaloblastic or pernicious anemia. Folic acid should not be given before a diagnosis has been fully established. Large and continuous dosage of folic acid may lower the blood concentration of Vitamin B12. The availability of folates may be inhibited by gastric acidity and the presence of ascorbate in the gastrointestinal tract. Metformin reduces intestinal absorption of folic acid and phenytoin may cause folate depletion. Folic acid does not correct folate deficiency due to dihydrofolate reductase inhibitors.