| 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 |
| Strict vegan diet |
| Vegetarian diet in pregnancy |
| Agents that block or impair absorption |
| 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 |
