Abstract

In temperate climates deficiency of vitamin B₁₂ is usually due to defective absorption, most commonly from lack of intrinsic factor as in Addisonian pernicious anaemia and after gastric surgery; occasionally the absorptive defect is caused by gastric carcinoma, resection, short-circuiting or disease of the ileum, including an abnormal bacterial flora, or by the fish tapeworm (Diphyllobothrium latum). The effectiveness of oral therapy remains debatable,¹ and, since the absorptive defect can rarely be corrected, injections of vitamin B₁₂ are virtually essential.