Inadequate secretion of intrinsic factor, resulting from lesions that destroy the gastric mucosa (ingestion of corrosives, extensive neoplasia), and a number of conditions associated with a variable degree of gastric atrophy (such as multiple sclerosis, certain endocrine disorders, iron deficiency, and subtotal gastrectomy)
Aside from that, vitamin B12 is an essential part of DNA synthesis, ensuring old and damaged cells are replaced with healthy ones
If the person appears nervous or tense, advise them to put their weight on the leg or arm that you aren't injecting to help relax the muscles
National Institutes of Health, Office of Dietary Supplements
Oral Supplements You can supplement vitamin B12 through either injections or oral supplements