Dependence on Cyanocobalamin Injections-Reply

In Reply.—  The comments by Drs Newbold and Tilkian underscore the fact that some aspects of vitamin B₁₂ metabolism in humans remain elusive despite 40 years of intensive study and thousands of publications.¹Dr Newbold points out that the hereditary defects of vitamin B₁₂ metabolism are not addressed in our article. Since these disorders have been considered rare and since they are often detected in infancy or childhood, it seemed unlikely that the adults in our study population would have been affected. However, two reports^2,3 suggesting that these inborn errors of metabolism may not manifest themselves until adolescence or adulthood raise the possibility that the clinical spectrum of these disorders is broader than previously suspected and that the potential benefits of cyanocobalamin therapy have been underestimated in certain patient populations.We agree with Dr Tilkian that much can be learned from routine laboratory tests. We specifically