Gastric abnormalities |
Autoantibodies to intrinsic factor or gastric parietal cells (ie, pernicious anemia) |
Gastrectomy/bariatric surgery |
Gastritis |
Autoimmune metaplastic atrophic gastritis |
Small bowel disease |
Malabsorption syndrome |
Ileal resection or bypass |
Inflammatory bowel disease (eg, Crohn disease) |
Celiac disease |
Bacterial overgrowth |
Blind loop |
Dibothriocephalus latus (fish tapeworm) |
Pancreatitis |
Pancreatic insufficiency |
Diet |
Breastfed infant of a mother with vitamin B12 deficiency |
Vegan diet |
Strict vegetarian diet |
Limited intake of animal protein during pregnancy or lactation |
Agents that block or impair absorption or metabolism |
Neomycin |
Biguanides (eg, metformin) |
Proton pump inhibitors (eg, omeprazole) |
Histamine 2 receptor antagonists (eg, cimetidine) |
Nitrous oxide (N2O) gas, used for anesthesia or recreationally |
Inherited transcobalamin II deficiency |
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