By comparison, oral B12 supplements are absorbed at roughly 1 to 5% in healthy adults, and even less if you have low intrinsic factor, atrophic gastritis, celiac disease, Crohn's, or a history of gastric bypass
Journal of Clinical Pharmacology, 63(12), 1121-1130
It is particularly common in individuals with certain risk factors, including those who have undergone gastric or small intestine resections, have inflammatory bowel disease, use metformin or proton pump inhibitors, follow a vegan diet, or are older than 75 years
Recommending unapproved treatments undermines this principle and puts patients at unnecessary risk