Gastric causes: gastrectomy, gastric resection, atrophic gastritis, H. pylori infection or congenital IF deficiency or abnormality. Inadequate dietary intake of vitamin B12 eg, a vegan diet. Intestinal causes eg, malabsorption, ileal resection, Crohn's disease affecting the ileum, chronic tropical sprue, HIV and any radiotherapy causing irradiation of the ileum. Drugs eg, colchicine, neomycin, ...