How should vitamin B12 be administered? I have read that injections are not necessary—even for gastrectomy patients. However, some of my patients return with instructions from a consultant, e.g., a hematologist/oncologist, that they are to have B12 injections for life. Can these patients take their B12 by mouth?
—Isaac Blum, MD, Bronx, N.Y.

During the 1950s and 1960s, researchers identified alternate pathways for passive absorption of B12. These pathways are independent of functioning intrinsic factor or an intact terminal ileum. While absorption may vary from individual to individual, approximately 1% of a large oral dose is absorbed passively (a large dose being defined as 300-1,000 µg). Multiple trials of oral B12 given to patients with B12 deficiency from a variety of causes have shown this strategy to be effective. For example, a randomized, controlled study showed that high-dose oral cyanocobalamin on a daily basis was “as effective as 1 mg administered IM on a monthly basis and may be superior” (Blood. 1998;92:1191-1198). Oral B12 replacement has been embraced for decades in countries such as Sweden, where about 80% of replacement is given via this method (J Intern Med. 1999;246:237-238). Despite experience and literature, some camps remain hesitant to fully embrace oral B12 replacement. This controversy is sometimes based on early fears that because of variable absorption, certain patients would not reach a therapeutic B12 level—a fear that is virtually nullified by use of large oral doses. Others cite compliance problems. Nonetheless, substantial evidence supports oral B12 as an effective form of replacement for patients with vitamin B12 deficiency. As always, patients should be followed for normalization of B12 levels, clinical improvement, and the potential for unmasked folic acid deficiency.
—Christopher Ruser, MD